xorg: add fglrxCompat parameter

pkgs/servers/x11/xorg/overrides.nix

···
     '';
   };
 
-  xorgserver = with xorg; attrs: attrs //
+  xorgserver = with xorg; attrs_passed:
+    # exchange attrs if fglrxCompat is set
+    let
+      attrs = if !args.fglrxCompat then attrs_passed else
+        with args; {
+          name = "xorg-server-1.17.4";
+          builder = ./builder.sh;
+          src = fetchurl {
+            url = mirror://xorg/individual/xserver/xorg-server-1.17.4.tar.bz2;
+            sha256 = "0mv4ilpqi5hpg182mzqn766frhi6rw48aba3xfbaj4m82v0lajqc";
+          };
+          buildInputs = [pkgconfig dri2proto dri3proto renderproto libdrm openssl libX11 libXau libXaw libxcb xcbutil xcbutilwm xcbutilimage xcbutilkeysyms xcbutilrenderutil libXdmcp libXfixes libxkbfile libXmu libXpm libXrender libXres libXt ];
+          meta.platforms = stdenv.lib.platforms.unix;
+        };
+
+    in attrs //
     (let
       version = (builtins.parseDrvName attrs.name).version;
       commonBuildInputs = attrs.buildInputs ++ [ xtrans ];

pkgs/top-level/all-packages.nix

···
     mesa = mesa_noglu;
     udev = if stdenv.isLinux then udev else null;
     libdrm = if stdenv.isLinux then libdrm else null;
+    fglrxCompat = false;
   } // { inherit xlibsWrapper; } );
 
   xwayland = callPackage ../servers/x11/xorg/xwayland.nix { };