Added support for LLVM/clang MacOSX

* Fixes issue #25.
* No need to install gcc on MacOSX. It works out of the box.
* Since OpenMP is still not ported to MacOSX (http://www.phoronix.com/scan.php?page=news_item&px=MTI2MjU) it runs in single core on mac, for now.
* Since @UPPMAX is down until friday (I've heard), I couldn't resist on implementing this :)