In case the configure script finds both gtk2 and gtk3 installed it
still prefers gtk2 over gtk3.  Prefer gtk3 instead.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170606105339.3613-2-kraxel@redhat.com