build: fix version number if we don't have git

The build should work from whithin released tar balls, git exports
or on systems where git is not available. This includes automated
download archives on github pages. That's why we need a more
complicated script "git-version-gen" (This one is derived from
twstools and git projects).

Additional notes.
 - git-version-gen is written for portability so it might look a
   bit circumstantially at some points.
 - If cmake had a "make dist" release target it should create a
   file named "version" to override all the git magic. We could
   also simply set DEF_VER correctly always before tagging a
   release.
 - The script can be extended to always update the version on
   make rathen than only on cmake (like it was originally in
   twstools project to work with automake).
Merge pull request #71 from rudimeier/git-version

build: fix version number if we don't have git