mesa: create a buildEnv that joins mesa (without glu) and glu

Since mesa-9.x, upstream has separated mesa and glu. @peti suggested using
buildEnv to create a mesa environment that acts like old mesa, which is what
this commit does.

pkgs/development/libraries/mesa-glu/default.nix

@@ -11,8 +11,6 @@ stdenv.mkDerivation rec {
   buildInputs = [ pkgconfig ];
   propagatedBuildInputs = [ mesa_noglu ];
 
-  passthru = { inherit (mesa_noglu) libdrm; inherit mesa_noglu; };
-
   meta = {
     description = "OpenGL utility library";
     homepage = http://cgit.freedesktop.org/mesa/glu/;