opengl: Do VirtualGL preload conditioning on VGL_DISPLAY

Preloading is done by doing a binary patch to the library to add it
as a dependency. The 2.3 series of glibc will let this be done with
just a wrapper that calls ld.so and with the --preload option.

pkgs/opengl/default.nix

@@ -3,7 +3,7 @@
 # Add a setup hook to the mesa_noglu package that automatically adds
 # a libvglfaker.so dependency to executables that depend on libGL.so.
 
-{ super, lib, buildEnv, makeSetupHook, file, bash }:
+{ super, lib, buildEnv, makeSetupHook, file, bash, xorg }:
 
 let
   autoVirtualGLHook =
@@ -27,7 +27,7 @@ let
       makeSetupHook {
         name = "insert-virtualgl";
         deps = [ file virtualglLib ];
-        substitutions = { inherit virtualglLib; };
+        substitutions = { inherit virtualglLib bash; inherit (xorg) libXext; };
       } ./insert-virtualgl.sh;
 
 in {