docs: don't require libmx-gtk when building libmx documentation
docs: don't require libmx-gtk when building libmx documentation

http://bugzilla.clutter-project.org/show_bug.cgi?id=2609

jonnylamb/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx