xlibs: replace occurrences by xorg

This seems to have been confusing people, using both xlibs and xorg, etc.
- Avoided renaming local (and different) xlibs binding in gcc*.
- Fixed cases where both xorg and xlibs were used.
Hopefully everything still works as before.

pkgs/games/zoom/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, perl, expat, xlibsWrapper, freetype }:
 
-# !!! assert freetype == xlibs.freetype
+# !!! assert freetype == xorg.freetype
 
 stdenv.mkDerivation {
   name = "zoom-1.1.5";