automake: recognize all-numeric MAJ.MIN.MICROa.ALPHA versions better.

automake: recognize all-numeric MAJ.MIN.MICROa.ALPHA versions better.

* HACKING: more doc on version numbers.
* configure.ac (AC_INIT): 1.17.0.91, since we want (I guess)
to retain odd numbers for development and even numbers for pretests.
* m4/amversion.m4 (AM_AUTOMAKE_VERSION): 1.17.0.91 (auto-update).
(AM_SET_CURRENT_AUTOMAKE_VERSION): likewise.
* lib/Automake/Options.pm (_process_option_list): recognize
four-part numeric versions as Automake options.
* lib/Automake/Version.pm (split): recognize four-part numeric
versions.
* t/pm/Version.pl (@tests, @bad_versions): update test
to check (not reject) four-part numeric versions.