configure should warn if libxml2 is not found.
libxml2 is needed by glapi/gen.
Fixes error during build in src/mapi/glapi/gen:
ImportError: No module named libxml2
NOTE: This is a candidate for the 9.0 branch.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31598
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
