mesa: use llvm6

pkgs/top-level/all-packages.nix

@@ -10520,6 +10520,7 @@ with pkgs;
     # makes it slower, but during runtime we link against just mesa_drivers
     # through /run/opengl-driver*, which is overriden according to config.grsecurity
     # grsecEnabled = true; # no more support in nixpkgs ATM
+    llvmPackages = llvmPackages_6;
   };
 
   mesa_glu =  callPackage ../development/libraries/mesa-glu { };