build: Skip Gtk.js test if GTK not detected
build: Skip Gtk.js test if GTK not detected

If GJS was not compiled with support for GTK, then there is no point in
running this test.

https://bugzilla.gnome.org/show_bug.cgi?id=772027

endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs