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/science/math/mathematica/9.nix

@@ -12,7 +12,7 @@
 , opencv
 , openssl
 , unixODBC
-, xlibs
+, xorg
 , zlib
 }:
 
@@ -51,7 +51,7 @@ stdenv.mkDerivation rec {
     opencv
     openssl
     unixODBC
-  ] ++ (with xlibs; [
+  ] ++ (with xorg; [
     libX11
     libXext
     libXtst