Accept gl linkage in hope of future offscreen mesa support.
Avoid linking with GLX though.

Change-Id: I4e666f60e74fe34075a8da9eeba95807cf8fe38c
Signed-off-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/13452Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>