autotools: use correct gl.pc LIBS when using glvnd

This is more of a hack, since glvnd itself should be providing the file.
Until that happens, ensure the libs is correctly set to -lGL

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 315c46cf)