Improve the beta number generation.

* autogen.sh: Add option --find-version
* configure.ac: Rework the setting of the mym4_ variables.

The old system was not too well defined if no release for a series has
yet been done. We now introduce a "PACKAGE-N.M-base" tag to solve
this problem. To keep the M4 code readable the GIT parsing has been
moved to ./autogen.sh.