OS X compatibility

Hi

NativeCall has some regression when it comes to locating library to load. In perfect world it should be just `is native ('libgd')` for all operating systems. But for now to make your code Mac OS X compatible I've used trick that recognizes system by VM configuration and applies correct library extension.

Tested under Mac OS X 10.7.4, with MacPorts package https://trac.macports.org/browser/trunk/dports/graphics/gd2 . Your synopsis worked fine and generated `test.png` image.

I've also fixed test so that it can be called from main project directory (default way of invoking prove).