[darwin.stdenv] Fix to work on Sierra

This reinstates the libSystem selective symbol export machinery we used
to have, but locks it to the symbols that were present in 10.11 and skips
the actual compiled code we put into that library in favor of the system
initialization code. That should make it more stable and less likely to
do weird stuff than the last time we did this.

pkgs/development/compilers/llvm/3.7/llvm.nix

@@ -35,6 +35,18 @@ in stdenv.mkDerivation rec {
 
   propagatedBuildInputs = [ ncurses zlib ];
 
+  # The goal here is to disable LLVM bindings (currently go and ocaml) regardless
+  # of whether the impure CMake search sheananigans find the compilers in global
+  # paths. This mostly exists because sandbox builds don't work very well on Darwin
+  # and sometimes you get weird behavior if CMake finds go in your system path.
+  # This would be far prettier if there were a CMake option to just disable bindings
+  # but from what I can tell, there isn't such a thing. The file in question only
+  # contains `if(WIN32)` conditions to check whether to disable bindings, so making
+  # those always succeed has the net effect of disabling all bindings.
+  prePatch = ''
+    substituteInPlace cmake/config-ix.cmake --replace "if(WIN32)" "if(1)"
+  '';
+
   # hacky fix: created binaries need to be run before installation
   preBuild = ''
     mkdir -p $out/