Fixed a few compiler issues in stb_vorbis

I've tried to get rid of anything that causes compiler warnings for the sake of a clean build, and I don't believe it's had any effect on functionality. Since GCC on Linux requires the -lm flag to include "math.h", I've added that as a #cgo preprocessor to add that when necessary. This has been tested on Windows 8 (64-bit) and Linux Mint 12 (32-bit).