Using git describe in Makefile

With using `git describe` in the Makefile it is easy to package the
application by just creating a new git tag.

Might be interesting for creating version strings:
https://wiki.debian.org/Punctuation

For example:
the checkout of the git tag `0.3.2rc1` will return exactly that git tag.
When developing further and creating a new tar.gz for distribution
it will return with the current head `0.3.2rc1-21-g8bef118`.
This string contains the last tag, number of commits after the last
tag and the current commit id.