This was introduced when the Cogl maintainers tried to move away from GLib. Since we always require it, we can just use `g_return_if_fail()` immediately. https://gitlab.gnome.org/GNOME/mutter/merge_requests/629
In cogl use cogl-config.h and in clutter use clutter-build-config.h. We can't use clutter-config.h in clutter because its already used and installed. https://bugzilla.gnome.org/show_bug.cgi?id=768976