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.