Add flag to disable hard errors for Clang builds on OS X.
Add flag to disable hard errors for Clang builds on OS X.

The Python on OS X was compiled with flags that are no longer supported
as of the latest versions, so any objects compiled with distutils are
failing.

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