rust-cpython: fix build for MacOSX

MacOSX needs special link flags. Quoting the README of rust-cpython:

  create a `.cargo/config` with the following content:

  ```
  [target.x86_64-apple-darwin]
  rustflags = [
    "-C", "link-arg=-undefined",
    "-C", "link-arg=dynamic_lookup",
  ]
  ```

This is tested with Python 2.7 (Anaconda install) and Python 3
(Homebrew install)

setup.py

@@ -1199,6 +1199,9 b' class RustExtension(Extension):'
         if sys.version_info[0] == 3 and self.py3_features is not None:
             cargocmd.extend(('--features', self.py3_features,
                              '--no-default-features'))
+        if sys.platform == 'darwin':
+            env['RUSTFLAGS'] = ("-C link-arg=-undefined "
+                                "-C link-arg=dynamic_lookup")
         try:
             subprocess.check_call(cargocmd, env=env, cwd=self.rustsrcdir)
         except OSError as exc: