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/tools/graphics/graphviz/2.32.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchurl, pkgconfig, libpng, libjpeg, expat, libXaw
-, yacc, libtool, fontconfig, pango, gd, xlibs, gts, gettext, cairo
+, yacc, libtool, fontconfig, pango, gd, xorg, gts, gettext, cairo
 }:
 
 stdenv.mkDerivation rec {
@@ -14,7 +14,7 @@ stdenv.mkDerivation rec {
   buildInputs =
     [ pkgconfig libpng libjpeg expat libXaw yacc libtool fontconfig
       pango gd gts
-    ] ++ stdenv.lib.optionals (xlibs != null) [ xlibs.xlibsWrapper xlibs.libXrender ]
+    ] ++ stdenv.lib.optionals (xorg != null) [ xorg.xlibsWrapper xorg.libXrender ]
     ++ stdenv.lib.optional (stdenv.system == "x86_64-darwin") gettext;
 
   CPPFLAGS = stdenv.lib.optionalString (stdenv.system == "x86_64-darwin") "-I${cairo}/include/cairo";
@@ -29,7 +29,7 @@ stdenv.mkDerivation rec {
       "--with-cgraph=no"
       "--with-sparse=no"
     ]
-    ++ stdenv.lib.optional (xlibs == null) "--without-x";
+    ++ stdenv.lib.optional (xorg == null) "--without-x";
 
   preBuild = ''
     sed -e 's@am__append_5 *=.*@am_append_5 =@' -i lib/gvc/Makefile