Compilation on OS X (Darwin)

I already opened an issue on [SourceForge](http://sourceforge.net/p/rhash/bugs/45/) but didn't get a response, so I assume the bugtracker there isn't used anymore.

I made some changes to the Makefile to support compilation on Mac OS X.
Since I don't know much about Makefiles please double check these changes, compilation does work on my MacBook though.

It would be great if this, or an adapted version, could be integrated into upstream so I can update the Homebrew Formula on Mac OS X to build the shared and static libraries too.