update full git version info at make time

There are still some things to think about before using this for even more projects. I just note them here to not forget about it.
- better build a minimal version.la?
- do we still need man page dependency on configure?
- rebuild man pages only on install and dist tagets?
- make more sure that man pages are not rebuilt when building from tar ball
- currently version_defs.h is in the dist (BUILT_SOURCES). It's not needed there.
- we could again use AC_INIT's version either
  * date based -just for fun
  * or again git-version-gen based to know whether we have a clean, bootstrapped build.