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/os-specific/linux/opengl/xorg-sys/default.nix

@@ -6,11 +6,11 @@
 # Of course, use of the driver in /usr/lib is highly impure.  But it
 # might actually work ;-)
 
-{stdenv, xlibs, expat, libdrm}:
+{stdenv, xorg, expat, libdrm}:
 
 stdenv.mkDerivation {
   name = "xorg-sys-opengl-3";
   builder = ./builder.sh;
-  neededLibs = [xlibs.libXxf86vm xlibs.libXext expat libdrm stdenv.cc.cc];
+  neededLibs = [xorg.libXxf86vm xorg.libXext expat libdrm stdenv.cc.cc];
 }