treewide: use llvm6 in some places

Not everything works on llvm7. It’s easiest to just provide the llvm6
as backup in this case.

pkgs/desktops/gnustep/default.nix

@@ -1,4 +1,4 @@
-{ pkgs, newScope }:
+{ pkgs, newScope, stdenv, llvmPackages_6 }:
 
 let
   callPackage = newScope self;
@@ -10,7 +10,9 @@ let
     gorm = callPackage ./gorm {};
     projectcenter = callPackage ./projectcenter {};
     system_preferences = callPackage ./systempreferences {};
-    libobjc = callPackage ./libobjc2 {};
+    libobjc = callPackage ./libobjc2 {
+      stdenv = if stdenv.cc.isClang then llvmPackages_6.stdenv else stdenv;
+    };
     make = callPackage ./make {};
     back = callPackage ./back {};
     base = callPackage ./base { giflib = pkgs.giflib_4_1; };