cc-wrapper: attempt to fix on darwin

This fixes evaluation at least. I'm unable to test this myself and noone steps up;
https://github.com/NixOS/nixpkgs/pull/7701#issuecomment-174448705
however, Hydra's macs are idling so let's employ them for that.

Changed files
+1 -1
pkgs
build-support
cc-wrapper
+1 -1
pkgs/build-support/cc-wrapper/default.nix
···
libc_dev = if nativeLibc then null else libc.dev or libc;
libc_lib = if nativeLibc then null else libc.out or libc;
cc_solib = cc.lib or cc;
-
binutils_bin = if nativeTools then null else binutils.bin or binutils;
+
binutils_bin = if nativeTools then "$binutils" else binutils.bin or binutils;
# The wrapper scripts use 'cat', so we may need coreutils.
coreutils_bin = if nativeTools then null else coreutils.bin or coreutils;
in