Add overridability to gnome

svn path=/nixpkgs/trunk/; revision=23773

pkgs/desktops/gnome-2.28/default.nix

···
 pkgs:
-
-rec {
+pkgs.makeOverridable
+(__overrides: rec {
+  inherit __overrides;
 
   inherit (pkgs.gtkLibs) glib pango atk gtk gtkmm;
 
···
     inherit gtkmm libglade;
   };
 
-}
+}) {}