release: distinguish major and minor releases

release: distinguish major and minor releases

This is mostly a preparatory change, in view of future
improvements in our release process.

* Makefile.am (determine_release_type): Also distinguish
between major and minor releases, with the help of ...
(stable_major_version_rx, stable_minor_version_rx): ... these
new macros.
(stable_version_rx): Remove, it's obsolete now.
(version_rx): Rename ...
(base_version_rx): ... like this, and adjust.
(print-release-type): New target; helps in debugging the code
that determines the release type.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>