Newer RHEL distributions have pkgconfig files split across three locations,
so add support for both share/pkgconfig and lib${LIBABISUFFIX}/pkgconfig if
LIBABISUFFIX is defined.
X.Org found in NetBSD-current.
Thanks a lot to all who helped, especially Matthias Scheler who did
repeated tests on Mac OS X and older versions of NetBSD to make sure the
support for those platforms wouldn't be broken (or at least, not fatally,
as I would still expect a few hiccups here and there, because there is
only so much one can test in such limited time).
On the infrastructure side, this branch brings pkgconfig-builtin.mk, in
order to write very easily new builtin.mk files. It can actually handle
more than just pkgconfig files, but it will provide a version if it finds
such a file. x11.builtin.mk has also been made more useful and now all
existing (and future!) native-X11-related builtin.mk files should include
it.
