fix: build on OSX was broken, because of abs(double) include cmath.

http://stackoverflow.com/questions/1374037/ambiguous-overload-call-to-absdouble

math.h is the outdated math library, which only contains abs(int),
the C++ version -- cmath.h includes the overload. Apparently GCC
and Windows already overload the abs() inside math.h, because this
only broke on OSX.