Dear developers,
Debian (and Ubuntu) are beginning to remove foo-config legacy scripts.
Already, xml2-config has been flagged for removal, with packages being
asked to switch to pkg-config.
This patch uses pkg-config's PKG_CHECK_MODULES macro to detect libxml2
or, if pkg-config is not available, falls back to xml2-confg.
The patch was created against the master branch of git.
I have built PostgreSQL and run `make check`. All 196 tests passed.
Hugh