Prevent concurrent use of GTK+ 2 and 3
Prevent concurrent use of GTK+ 2 and 3

If we detect a gtk3-only symbol, error out early.

Distrotech/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Unity-Technologies/gtk,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk