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

configure.in

@@ -589,6 +589,10 @@ else
 fi
 AC_SUBST(TAS)
 
+#
+# Set up pkg_config in case we need it below
+#
+PKG_PROG_PKG_CONFIG
 
 #
 # Automatic dependency tracking
@@ -845,26 +849,41 @@ AC_SUBST(UUID_EXTRA_OBJS)
 #
 # XML
 #
+AC_MSG_CHECKING([whether to build with XML support])
 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)])])
+AC_MSG_RESULT([$with_libxml])
+AC_SUBST(with_libxml)
 
 if test "$with_libxml" = yes ; then
-  PGAC_PATH_PROGS(XML2_CONFIG, xml2-config)
-  if test -n "$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 $pgac_option";;
-      esac
-    done
+  # Check pkg-config, then xml2-config.  But for backwards compatibility,
+  # setting XML2_CONFIG overrides pkg-config.
+  AC_ARG_VAR(XML2_CONFIG, [path to xml2-config utility])dnl
+  have_libxml2_pkg_config=no
+  if test -z "$XML2_CONFIG" -a -n "$PKG_CONFIG"; then
+    PKG_CHECK_MODULES(XML2, [libxml-2.0 >= 2.6.23],
+                      [have_libxml2_pkg_config=yes], [# do nothing])
   fi
+  if test "$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)
 
 #
 # XSLT