Disable just gdk-pixbuf test when unavailable
Submitted by Tom Schoonjans
Link to original bug (#754694)
Description
When compiling gom on a system without gdk-pixbuf I noticed that configure should have automatically disabled the testsuite but didn't, which resulted in a compile-time error due to a missing gdk-pixbuf header. This patch introduces ENABLE_GLIB_TEST in tests/Makefile.include, fixing the aforementioned issue.