Set _USE_MATH_DEFINES for MSVC builds.
Add this define to lib/os.h and test/util.c so M_PI is available when building with Microsoft's math.h. On other systems, it's typically enabled by default, e.g. through __USE_XOPEN or __USE_GNU. We already had a fallback definition of M_PI in os.h which was used by the core library, but this should be more consistent. Based on a patch by Ozkan Sezer.