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.

What is still missing is better handling of dependencies so that not all files will be rebuilt always.

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.