Add warnings about environment variables
Add warnings about environment variables

These variables all get read by GTK 3 as well, which makes
them dangerous at best.

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