Lower priority of the configure option --with-libraries causes wrong lib resolution - Mailing list pgsql-bugs

Hello,
I am having issues building Postgres due to the `./configure` script. I 
found a workaround but I would appreciate a more reliable fix.

I use Arch Linux 64bit and want to build Postgres with the latest libicu 
version which is version 77.1. Unfortunately the Arch package is still 
at version 76 currently [0]. To solve this, I built libicu 77.1 locally. 
I also built the latest libxml2 with the latest libicu to ensure compat. 
This means that I have a local directory with shared objects and include 
files for libicu 77. Let's say that these dependencies are in 
`/postgres-deps/lib` and `/postgres-deps/include`. I also still have the 
system-level ICU version 76 present in `/usr/lib`.

My goal is to build Postgres with the libicu version from `libicuuc.so` 
(77) instead of the one from `/usr/lib` (76). Both directories contain a 
file named `libicuuc.so`.


To achieve it, I called the `configure` script with the arguments 
`--with-libraries` and `--with-includes`, as such: `./configure 
--with-libraries=/postgres-deps/lib 
--with-includes=/postgres-deps/include ...` (+ some extra `--with` flags 
to enable features, irrelevant to this issue I think).

I then initiate the build with `LD_LIBRARY_PATH="/postgres-deps/lib " 
make all`. The C compilation into object code succeeds using the libicu 
77 includes, but then it fails to link with many errors such as:

```
/usr/bin/ld: commands/collationcmds.o: in function `get_icu_locale_comment':
collationcmds.c:(.text+0x1a20): undefined reference to 
`uloc_getDisplayName_77'
```

The failing command is fairly long so I shortened it to focus on the 
main part:

```

gcc [...COMPILER_FLAGS] [...OBJECT_FILES] -L../../src/port 
-L../../src/common  -L/usr/lib -L/postgres-deps/lib -Wl,--as-needed  
-Wl,--export-dynamic -lzstd -llz4 -lxslt -lxml2 -lpam -lssl -lcrypto 
-lgssapi_krb5 -lz -lm -lldap -licui18n -licuuc -lsystemd -o postgres
```

In particular, notice that the lib locations give priority to the system 
directory instead of the `--with-libraries` directory that I passed to 
`./configure`: `-L/usr/lib  -L/postgres-deps/lib`. Since the command 
requests `-licuuc`, the file `/usr/lib/libicuuc.so` (version 76) is 
matched first and the file `/usr/lib/libicuuc.so` (77) is ignored.


Swapping the two flags so the order is `-L/postgres-deps/lib -L/usr/lib` 
fixes the build. I consider it very surprising that that libraries 
passed with `--with-libraries` have lower priority, I would expect 
explicitly requested libraries to have the highest priority.


I've searched a bit to find where both locations are inserted.

First, `/usr/lib` is added in the clang configuration [1]. It is 
retrieved from `/usr/bin/llvm-config --ldflags`.

Second, the `--with-libraries` directories are collected into 
`$LIBDIRS`, with a check to verify that the dirs exists, this adds 
`/postgres-deps/lib` [2].

Finally, $LIBDIRS is _appended at the end of LDFLAGS_.using 
`LDFLAGS="$LDFLAGS $LIBDIRS"` [3].


My workaround is to update the configure script to instead prepend 
`LIBDIRS` at the start of `LDFLAGS` using `LDFLAGS="$LIBDIRS $LDFLAGS"`. 
With this change, the lib dir order is `-L/postgres-deps/lib -L/usr/lib` 
and the version 77 of libicuuc.so is picked by the linker. The build 
completes and I get a fully functional Postgres (at least it passes the 
test suite of my application depending on it).

My expectation that moving the lib directories passed using 
`--with-libraries` before any other linker flags makes sense, however 
I'm not an expert in this area and I guess that there may be situations 
where having the CLI libs first may cause issues. I can send a patch 
swapping the application order or `LIBDIRS` as described in my 
workaround if it makes sense. If there is a better solution to give 
higher priority to the libs passed through the configure CLI, I would 
gladly use it.


[0]: https://archlinux.org/packages/core/x86_64/icu/

[1]: 
https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L5197

[2]: 
https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L8104

[3]: 
https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L9824


Thank you in advance,
Charles "demurgos" Samborski