add is{GNU,Clang} attrs to cc-wrapper.

These will be more pleasant to use than the existing

    (cc.cc.isClang or false)

nonsense we currently do.

pkgs/build-support/cc-wrapper/default.nix

@@ -10,6 +10,7 @@
 , zlib ? null, extraPackages ? []
 , dyld ? null # TODO: should this be a setup-hook on dyld?
 , setupHook ? ./setup-hook.sh
+, isGNU ? false, isClang ? false
 }:
 
 with stdenv.lib;
@@ -41,7 +42,7 @@ stdenv.mkDerivation {
   # The wrapper scripts use 'cat', so we may need coreutils.
   coreutils = if nativeTools then null else coreutils;
 
-  passthru = { inherit nativeTools nativeLibc nativePrefix; };
+  passthru = { inherit nativeTools nativeLibc nativePrefix isGNU isClang; };
 
   buildCommand =
     ''