Add support for MacOS builds.
Add support for MacOS builds.

rjpower/py-leveldb,rjpower/py-leveldb,baverman/py-leveldb,rjpower/py-leveldb,baverman/py-leveldb,baverman/py-leveldb,rjpower/py-leveldb,baverman/py-leveldb,baverman/py-leveldb,baverman/py-leveldb