rudimental support of mac os x (aka Darwin)

For now it uses a custum path used by a widely used package magager called
MacPorts for the mp4v2 library.

Will probably add /usr/lib for those who want to compile it themselves later.
Right now I'm to tired to implement _and_ test if it works properly!

Tested the commit with an .m4b file I bought at a bitten fruit. :-)