Update Makefile

Siehe: https://stackoverflow.com/questions/16344445/im-using-math-h-and-the-library-link-lm-but-undefined-reference-to-pow-st

und 

https://stackoverflow.com/questions/1033898/why-do-you-have-to-link-the-math-library-in-c
Merge pull request #1 from Lechindianer/master

Update Makefile