giflib: don't link against libx11, don't depend on libsm

libsm wasn't used unless libx11 was built before giflib. Therefore
drop the dependency on libsm and disable x11 unconditionally like
Debian does.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>