Fix python_cpp test on Mac. Link staticly when building extension, so that the extension doesn't require installing protobuf library. (#2232)

diff --git a/python/tox.ini b/python/tox.ini
index cf8d540..1600db2 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -12,7 +12,7 @@
 commands =
     python setup.py -q build_py
     python: python setup.py -q build
-    cpp: python setup.py -q build --cpp_implementation --warnings_as_errors
+    cpp: python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension
     python: python setup.py -q test -q
     cpp: python setup.py -q test -q --cpp_implementation
     python: python setup.py -q test_conformance