Makefile enhancements; embed Git version

This addresses issue #7 (embed Git version), though it doesn't actually yet write the version anywhere, it merely embeds the version so that it's accessible with Version_GetGitVersion(). Perhaps at some point there will be a function for 'human readable' version, such as one based on tags (git describe --tags would work for this; in fact, perhaps even this version could just return that).

Also OBJS is now generated from the list of source files (so relates to #12) and dealing with BUILD_DIR generation is now handled better. The BUILD_DIR fix fixes the problem of always recompiling everything.

The generated version file does not get generated when building with the LPCXpresso Eclipse environment; I'm not sure if that's something people still want to support, though.