m4: Fix detection of gpgrt's libdir.

* src/gpg-error.m4 (GPGRT_CONFIG): Handle the case where none of the
system lib directories contain a pkgconfig subdirectory.

When we look for gpgrt_libdir, there's a corner case if we had been able
to obtain system libdirs (using cc -print-search-dirs) *but* none of
those system libdirs happen to contain a valid pkgconfig subdirectory
(which may be unlikely but can and does happen when cross-compiling).

We do test for the case where we have not obtained any system libdir at
all, in which case we fallback to ${gpgrt_prefix}/lib
(possible_libdir1), but we do not test if the list of libdir
candidates is reduced to nothing after we have eliminated all the
libdirs that do not contain a pkgconfig subdirectory.

This patch adds a test for this precise case.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>