Re: [PATCH] Use PKG_CHECK_MODULES to detect the libxml2 library - Mailing list pgsql-hackers

I poked at this issue a bit more and realized that Hugh's patch
flat-out breaks building --with-libxml in environments without
pkg-config, because PKG_CHECK_MODULES just gives up and dies
if there's no pkg-config (as we'd already found out in connection
with ICU).  That won't win us any friends, so the attached revision
doesn't call PKG_CHECK_MODULES unless we found pkg-config.  I also
concluded that if the user has set XML2_CONFIG, it's pretty clear
that her intent is to use whatever that is pointing at, so we should
not use pkg-config in that case either.

Also, I'd been going back and forth about whether it was worth
documenting XML2_CFLAGS/XML2_LIBS, but I realized that use of
PKG_CHECK_MODULES(XML2, ...) basically forces the issue for us:
it does AC_ARG_VAR on them, which puts them into configure's
--help output and makes configure picky about caching them.
So we can't really pretend they're boring implementation detail.

So the attached mostly adopts Peter's old suggested docs, but
I added discussion of XML2_CFLAGS/XML2_LIBS and dropped the mention
of forcing matters with --with-libs/--with-libraries (not because
that doesn't work anymore but because it seemed like we were offering
quite enough alternatives already).

I'd originally thought that we might back-patch this, but I'm now of
the opinion that we probably should not.  If pkg-config is present,
this can change the default behavior about where we get libxml from,
which seems like something not to do in minor releases.  (OTOH, it'd
only matter if the default pkg-config choice is different from the
default xml2-config choice, so maybe the risk of breakage is small
enough to be acceptable?)

            regards, tom lane

diff --git a/configure b/configure
index 1a0aca9..253df29 100755
--- a/configure
+++ b/configure
@@ -703,6 +703,8 @@ with_zlib
 with_system_tzdata
 with_libxslt
 with_libxml
+XML2_LIBS
+XML2_CFLAGS
 XML2_CONFIG
 UUID_EXTRA_OBJS
 with_uuid
@@ -719,13 +721,13 @@ with_perl
 with_tcl
 ICU_LIBS
 ICU_CFLAGS
-PKG_CONFIG_LIBDIR
-PKG_CONFIG_PATH
-PKG_CONFIG
 with_icu
 enable_thread_safety
 INCLUDES
 autodepend
+PKG_CONFIG_LIBDIR
+PKG_CONFIG_PATH
+PKG_CONFIG
 TAS
 GCC
 CPP
@@ -887,6 +889,8 @@ PKG_CONFIG_LIBDIR
 ICU_CFLAGS
 ICU_LIBS
 XML2_CONFIG
+XML2_CFLAGS
+XML2_LIBS
 LDFLAGS_EX
 LDFLAGS_SL
 PERL
@@ -1589,6 +1593,8 @@ Some influential environment variables:
   ICU_CFLAGS  C compiler flags for ICU, overriding pkg-config
   ICU_LIBS    linker flags for ICU, overriding pkg-config
   XML2_CONFIG path to xml2-config utility
+  XML2_CFLAGS C compiler flags for XML2, overriding pkg-config
+  XML2_LIBS   linker flags for XML2, overriding pkg-config
   LDFLAGS_EX  extra linker flags for linking executables only
   LDFLAGS_SL  extra linker flags for linking shared libraries only
   PERL        Perl program
@@ -7153,6 +7159,129 @@ else
 fi


+#
+# Set up pkg_config in case we need it below
+#
+
+
+
+
+
+
+
+if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
+    if test -n "$ac_tool_prefix"; then
+  # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.
+set dummy ${ac_tool_prefix}pkg-config; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_PKG_CONFIG+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $PKG_CONFIG in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_PKG_CONFIG="$PKG_CONFIG" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+PKG_CONFIG=$ac_cv_path_PKG_CONFIG
+if test -n "$PKG_CONFIG"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PKG_CONFIG" >&5
+$as_echo "$PKG_CONFIG" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+fi
+if test -z "$ac_cv_path_PKG_CONFIG"; then
+  ac_pt_PKG_CONFIG=$PKG_CONFIG
+  # Extract the first word of "pkg-config", so it can be a program name with args.
+set dummy pkg-config; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_ac_pt_PKG_CONFIG+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $ac_pt_PKG_CONFIG in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_ac_pt_PKG_CONFIG="$ac_pt_PKG_CONFIG" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_ac_pt_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+ac_pt_PKG_CONFIG=$ac_cv_path_ac_pt_PKG_CONFIG
+if test -n "$ac_pt_PKG_CONFIG"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_pt_PKG_CONFIG" >&5
+$as_echo "$ac_pt_PKG_CONFIG" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+  if test "x$ac_pt_PKG_CONFIG" = x; then
+    PKG_CONFIG=""
+  else
+    case $cross_compiling:$ac_tool_warned in
+yes:)
+{ $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: using cross tools not prefixed with host triplet" >&5
+$as_echo "$as_me: WARNING: using cross tools not prefixed with host triplet" >&2;}
+ac_tool_warned=yes ;;
+esac
+    PKG_CONFIG=$ac_pt_PKG_CONFIG
+  fi
+else
+  PKG_CONFIG="$ac_cv_path_PKG_CONFIG"
+fi
+
+fi
+if test -n "$PKG_CONFIG"; then
+    _pkg_min_version=0.9.0
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking pkg-config is at least version $_pkg_min_version" >&5
+$as_echo_n "checking pkg-config is at least version $_pkg_min_version... " >&6; }
+    if $PKG_CONFIG --atleast-pkgconfig-version $_pkg_min_version; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+    else
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+        PKG_CONFIG=""
+    fi
+fi

 #
 # Automatic dependency tracking
@@ -7321,126 +7450,6 @@ $as_echo "$with_icu" >&6; }

 if test "$with_icu" = yes; then

-
-
-
-
-
-
-if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
-    if test -n "$ac_tool_prefix"; then
-  # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.
-set dummy ${ac_tool_prefix}pkg-config; ac_word=$2
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
-$as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_path_PKG_CONFIG+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  case $PKG_CONFIG in
-  [\\/]* | ?:[\\/]*)
-  ac_cv_path_PKG_CONFIG="$PKG_CONFIG" # Let the user override the test with a path.
-  ;;
-  *)
-  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
-for as_dir in $PATH
-do
-  IFS=$as_save_IFS
-  test -z "$as_dir" && as_dir=.
-    for ac_exec_ext in '' $ac_executable_extensions; do
-  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_path_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
-    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
-    break 2
-  fi
-done
-  done
-IFS=$as_save_IFS
-
-  ;;
-esac
-fi
-PKG_CONFIG=$ac_cv_path_PKG_CONFIG
-if test -n "$PKG_CONFIG"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PKG_CONFIG" >&5
-$as_echo "$PKG_CONFIG" >&6; }
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-
-fi
-if test -z "$ac_cv_path_PKG_CONFIG"; then
-  ac_pt_PKG_CONFIG=$PKG_CONFIG
-  # Extract the first word of "pkg-config", so it can be a program name with args.
-set dummy pkg-config; ac_word=$2
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
-$as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_path_ac_pt_PKG_CONFIG+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  case $ac_pt_PKG_CONFIG in
-  [\\/]* | ?:[\\/]*)
-  ac_cv_path_ac_pt_PKG_CONFIG="$ac_pt_PKG_CONFIG" # Let the user override the test with a path.
-  ;;
-  *)
-  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
-for as_dir in $PATH
-do
-  IFS=$as_save_IFS
-  test -z "$as_dir" && as_dir=.
-    for ac_exec_ext in '' $ac_executable_extensions; do
-  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_path_ac_pt_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
-    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
-    break 2
-  fi
-done
-  done
-IFS=$as_save_IFS
-
-  ;;
-esac
-fi
-ac_pt_PKG_CONFIG=$ac_cv_path_ac_pt_PKG_CONFIG
-if test -n "$ac_pt_PKG_CONFIG"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_pt_PKG_CONFIG" >&5
-$as_echo "$ac_pt_PKG_CONFIG" >&6; }
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-  if test "x$ac_pt_PKG_CONFIG" = x; then
-    PKG_CONFIG=""
-  else
-    case $cross_compiling:$ac_tool_warned in
-yes:)
-{ $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: using cross tools not prefixed with host triplet" >&5
-$as_echo "$as_me: WARNING: using cross tools not prefixed with host triplet" >&2;}
-ac_tool_warned=yes ;;
-esac
-    PKG_CONFIG=$ac_pt_PKG_CONFIG
-  fi
-else
-  PKG_CONFIG="$ac_cv_path_PKG_CONFIG"
-fi
-
-fi
-if test -n "$PKG_CONFIG"; then
-    _pkg_min_version=0.9.0
-    { $as_echo "$as_me:${as_lineno-$LINENO}: checking pkg-config is at least version $_pkg_min_version" >&5
-$as_echo_n "checking pkg-config is at least version $_pkg_min_version... " >&6; }
-    if $PKG_CONFIG --atleast-pkgconfig-version $_pkg_min_version; then
-        { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
-$as_echo "yes" >&6; }
-    else
-        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-        PKG_CONFIG=""
-    fi
-fi
-
 pkg_failed=no
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for icu-uc icu-i18n" >&5
 $as_echo_n "checking for icu-uc icu-i18n... " >&6; }
@@ -8161,7 +8170,103 @@ fi


 if test "$with_libxml" = yes ; then
-  if test -z "$XML2_CONFIG"; then
+    # For backwards compatibility, setting XML2_CONFIG overrides pkg_config
+  have_libxml2_pkg_config=no
+  if test -z "$XML2_CONFIG" -a -n "$PKG_CONFIG"; then
+
+pkg_failed=no
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for libxml-2.0 >= 2.6.23" >&5
+$as_echo_n "checking for libxml-2.0 >= 2.6.23... " >&6; }
+
+if test -n "$XML2_CFLAGS"; then
+    pkg_cv_XML2_CFLAGS="$XML2_CFLAGS"
+ elif test -n "$PKG_CONFIG"; then
+    if test -n "$PKG_CONFIG" && \
+    { { $as_echo "$as_me:${as_lineno-$LINENO}: \$PKG_CONFIG --exists --print-errors \"libxml-2.0 >= 2.6.23\""; } >&5
+  ($PKG_CONFIG --exists --print-errors "libxml-2.0 >= 2.6.23") 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; then
+  pkg_cv_XML2_CFLAGS=`$PKG_CONFIG --cflags "libxml-2.0 >= 2.6.23" 2>/dev/null`
+              test "x$?" != "x0" && pkg_failed=yes
+else
+  pkg_failed=yes
+fi
+ else
+    pkg_failed=untried
+fi
+if test -n "$XML2_LIBS"; then
+    pkg_cv_XML2_LIBS="$XML2_LIBS"
+ elif test -n "$PKG_CONFIG"; then
+    if test -n "$PKG_CONFIG" && \
+    { { $as_echo "$as_me:${as_lineno-$LINENO}: \$PKG_CONFIG --exists --print-errors \"libxml-2.0 >= 2.6.23\""; } >&5
+  ($PKG_CONFIG --exists --print-errors "libxml-2.0 >= 2.6.23") 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; then
+  pkg_cv_XML2_LIBS=`$PKG_CONFIG --libs "libxml-2.0 >= 2.6.23" 2>/dev/null`
+              test "x$?" != "x0" && pkg_failed=yes
+else
+  pkg_failed=yes
+fi
+ else
+    pkg_failed=untried
+fi
+
+
+
+if test $pkg_failed = yes; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+
+if $PKG_CONFIG --atleast-pkgconfig-version 0.20; then
+        _pkg_short_errors_supported=yes
+else
+        _pkg_short_errors_supported=no
+fi
+        if test $_pkg_short_errors_supported = yes; then
+            XML2_PKG_ERRORS=`$PKG_CONFIG --short-errors --print-errors --cflags --libs "libxml-2.0 >= 2.6.23" 2>&1`
+        else
+            XML2_PKG_ERRORS=`$PKG_CONFIG --print-errors --cflags --libs "libxml-2.0 >= 2.6.23" 2>&1`
+        fi
+    # Put the nasty error message in config.log where it belongs
+    echo "$XML2_PKG_ERRORS" >&5
+
+    as_fn_error $? "Package requirements (libxml-2.0 >= 2.6.23) were not met:
+
+$XML2_PKG_ERRORS
+
+Consider adjusting the PKG_CONFIG_PATH environment variable if you
+installed software in a non-standard prefix.
+
+Alternatively, you may set the environment variables XML2_CFLAGS
+and XML2_LIBS to avoid the need to call pkg-config.
+See the pkg-config man page for more details." "$LINENO" 5
+elif test $pkg_failed = untried; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+    { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "The pkg-config script could not be found or is too old.  Make sure it
+is in your PATH or set the PKG_CONFIG environment variable to the full
+path to pkg-config.
+
+Alternatively, you may set the environment variables XML2_CFLAGS
+and XML2_LIBS to avoid the need to call pkg-config.
+See the pkg-config man page for more details.
+
+To get pkg-config, see <http://pkg-config.freedesktop.org/>.
+See \`config.log' for more details" "$LINENO" 5; }
+else
+    XML2_CFLAGS=$pkg_cv_XML2_CFLAGS
+    XML2_LIBS=$pkg_cv_XML2_LIBS
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+    have_libxml2_pkg_config=yes
+fi
+  fi
+  if test "$have_libxml2_pkg_config" = no ; then
+    if test -z "$XML2_CONFIG"; then
   for ac_prog in xml2-config
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
@@ -8216,17 +8321,21 @@ $as_echo "$XML2_CONFIG" >&6; }
 fi

     if test -n "$XML2_CONFIG"; then
-    for pgac_option in `$XML2_CONFIG --cflags`; do
-      case $pgac_option in
-        -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
-      esac
-    done
-    for pgac_option in `$XML2_CONFIG --libs`; do
-      case $pgac_option in
-        -L*) LDFLAGS="$LDFLAGS $pgac_option";;
-      esac
-    done
+      XML2_CFLAGS=`$XML2_CONFIG --cflags`
+      XML2_LIBS=`$XML2_CONFIG --libs`
+    fi
   fi
+  # Note the user could also set XML2_CFLAGS/XML2_LIBS directly
+  for pgac_option in $XML2_CFLAGS; do
+    case $pgac_option in
+      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+    esac
+  done
+  for pgac_option in $XML2_LIBS; do
+    case $pgac_option in
+      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+    esac
+  done
 fi


diff --git a/configure.in b/configure.in
index b27708e..d4e7a87 100644
--- a/configure.in
+++ b/configure.in
@@ -667,6 +667,10 @@ else
 fi
 AC_SUBST(TAS)

+#
+# Set up pkg_config in case we need it below
+#
+PKG_PROG_PKG_CONFIG

 #
 # Automatic dependency tracking
@@ -931,20 +935,31 @@ PGAC_ARG_BOOL(with, libxml, no, [build with XML support],
               [AC_DEFINE([USE_LIBXML], 1, [Define to 1 to build with XML support. (--with-libxml)])])

 if test "$with_libxml" = yes ; then
-  PGAC_PATH_PROGS(XML2_CONFIG, xml2-config)
   AC_ARG_VAR(XML2_CONFIG, [path to xml2-config utility])dnl
-  if test -n "$XML2_CONFIG"; then
-    for pgac_option in `$XML2_CONFIG --cflags`; do
-      case $pgac_option in
-        -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
-      esac
-    done
-    for pgac_option in `$XML2_CONFIG --libs`; do
-      case $pgac_option in
-        -L*) LDFLAGS="$LDFLAGS $pgac_option";;
-      esac
-    done
+  # For backwards compatibility, setting XML2_CONFIG overrides pkg_config
+  have_libxml2_pkg_config=no
+  if test -z "$XML2_CONFIG" -a -n "$PKG_CONFIG"; then
+    PKG_CHECK_MODULES(XML2, [libxml-2.0 >= 2.6.23],
+                      [have_libxml2_pkg_config=yes])
+  fi
+  if test "$have_libxml2_pkg_config" = no ; then
+    PGAC_PATH_PROGS(XML2_CONFIG, xml2-config)
+    if test -n "$XML2_CONFIG"; then
+      XML2_CFLAGS=`$XML2_CONFIG --cflags`
+      XML2_LIBS=`$XML2_CONFIG --libs`
+    fi
   fi
+  # Note the user could also set XML2_CFLAGS/XML2_LIBS directly
+  for pgac_option in $XML2_CFLAGS; do
+    case $pgac_option in
+      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+    esac
+  done
+  for pgac_option in $XML2_LIBS; do
+    case $pgac_option in
+      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+    esac
+  done
 fi

 AC_SUBST(with_libxml)
diff --git a/doc/src/sgml/installation.sgml b/doc/src/sgml/installation.sgml
index cc242dc..0326ac4 100644
--- a/doc/src/sgml/installation.sgml
+++ b/doc/src/sgml/installation.sgml
@@ -1122,16 +1122,26 @@ build-postgresql:
         </para>

         <para>
-         Libxml installs a program <command>xml2-config</command> that
-         can be used to detect the required compiler and linker
-         options.  PostgreSQL will use it automatically if found.  To
-         specify a libxml installation at an unusual location, you can
-         either set the environment variable
-         <envar>XML2_CONFIG</envar> to point to the
-         <command>xml2-config</command> program belonging to the
-         installation, or use the options
-         <option>--with-includes</option> and
-         <option>--with-libraries</option>.
+         To detect the required compiler and linker options, PostgreSQL will
+         query <command>pkg-config</command>, if that is installed and knows
+         about libxml2.  Otherwise the program <command>xml2-config</command>,
+         which is installed by libxml, will be used if it is found.  Use
+         of <command>pkg-config</command> is preferred, because it can deal
+         with multi-architecture installations better.
+        </para>
+
+        <para>
+         To use a libxml installation that is in an unusual location, you
+         can set <command>pkg-config</command>-related environment
+         variables (see its documentation), or set the environment variable
+         <envar>XML2_CONFIG</envar> to point to
+         the <command>xml2-config</command> program belonging to the libxml
+         installation, or set the variables <envar>XML2_CFLAGS</envar>
+         and <envar>XML2_LIBS</envar>.  (If <command>pkg-config</command> is
+         installed, then to override its idea of where libxml is you must
+         either set <envar>XML2_CONFIG</envar> or set
+         both <envar>XML2_CFLAGS</envar> and <envar>XML2_LIBS</envar> to
+         nonempty strings.)
         </para>
        </listitem>
       </varlistentry>