NMake Makefiles: Fix builds from release tarballs

This will avoid re-generating atkmm.rc and atkmmconfig.h unneedingly, and
ensure that we do have the version info in those files when we do need to
generate them.

Also streamline the process so that these files are generated as part of the
'all' target, so there is no need to run the 'prep-git-build' target explicitly