Use pkg-config, if available, to locate libxml2 during configure.

If pkg-config is installed and knows about libxml2, use its information
rather than asking xml2-config.  Otherwise proceed as before.  This
patch allows "configure --with-libxml" to succeed on platforms that
have pkg-config but not xml2-config, which is likely to soon become
a typical situation.

The old mechanism can be forced by setting XML2_CONFIG explicitly
(hence, build processes that were already doing so will certainly
not need adjustment).  Also, it's now possible to set XML2_CFLAGS
and XML2_LIBS explicitly to override both programs.

There is a small risk of this breaking existing build processes,
if there are multiple libxml2 installations on the machine and
pkg-config disagrees with xml2-config about which to use.  The
only case where that seems really likely is if a builder has tried
to select a non-default xml2-config by putting it early in his PATH
rather than setting XML2_CONFIG.  Plan to warn against that in the
minor release notes.

Back-patch to v10; before that we had no pkg-config infrastructure,
and it doesn't seem worth adding it for this.

Hugh McMaster and Tom Lane; Peter Eisentraut also made an earlier
attempt at this, from which I lifted most of the docs changes.

Discussion: https://postgr.es/m/CAN9BcdvfUwc9Yx5015bLH2TOiQ-M+t_NADBSPhMF7dZ=pLa_iw@mail.gmail.com

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