Re-add the glx compile test to configure.  We can't use pkg-config to
probe for glx, and as long as milkymist-tmu2 privately uses glx (due to
opengl infrastructure in qemu not being ready yet) we must continue to
test for glx to avoid build failures.

Reported-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>