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/applications/version-management/git-and-tools/default.nix

@@ -51,13 +51,13 @@ rec {
 
   qgit = import ./qgit {
     inherit fetchurl stdenv;
-    inherit (xlibs) libXext libX11;
+    inherit (xorg) libXext libX11;
     qt = qt4;
   };
 
   qgitGit = import ./qgit/qgit-git.nix {
     inherit fetchurl sourceFromHead stdenv;
-    inherit (xlibs) libXext libX11;
+    inherit (xorg) libXext libX11;
     qt = qt4;
   };