VERSION obtained from source instead of the previous git tag.

Drawback: The version string is no longer a valid git identifier.

For this reason the 'g' short hash prefix has been removed.

Exception: When building directly from a tag this behaves exactly like the previous behavior.

This allows formatting release versions with precision     i.e. v0.9.2
This also allows arbitrary topicbranch names               i.e. v0.9.1-glibc-compat