python/grpcio: fix darwin build

grpcio currently does not build on Darwin (10.13.6). Due to the
following issues:

* ar is invoked with incorrect flags (#33765).
* libtool cannot be found, with a libtool dependency, with libtool
  the option '-no_warning_for_no_symbols' is not recognized.
* the module build cannot find boringssl that is packaged with
  python-grpcio when pkgconfig is not installed (grpc/#10058).

(cherry picked from commit 4beb94d)

pkgs/development/python-modules/grpcio/default.nix

@@ -1,6 +1,7 @@
-{ stdenv, buildPythonPackage, fetchPypi, lib
-, six, protobuf3_5, enum34, futures, isPy26, isPy27, isPy34 }:
+{ stdenv, buildPythonPackage, fetchPypi, lib, darwin
+, six, protobuf3_5, enum34, futures, isPy26, isPy27, isPy34, pkgconfig }:
 
+with stdenv.lib;
 buildPythonPackage rec {
   pname = "grpcio";
   version = "1.9.1";
@@ -14,6 +15,10 @@ buildPythonPackage rec {
                         ++ lib.optionals (isPy26 || isPy27 || isPy34) [ enum34 ]
                         ++ lib.optionals (isPy26 || isPy27) [ futures ];
 
+  nativeBuildInputs = [ pkgconfig ] ++ optional stdenv.isDarwin darwin.cctools;
+
+  preBuild = optionalString stdenv.isDarwin "unset AR";
+
   meta = with stdenv.lib; {
     description = "HTTP/2-based RPC framework";
     license = lib.licenses.bsd3;