Fix autopoint version detection again.

Both copies of the autopoint version detector (one in autoreconf and
one in tests/local.at) should now correctly recognize autopoint
0.2[34].(>=2) as supporting AM_GNU_GETTEXT without a version
requirement, as well as 0.25.1 and up.  The logic has been revised
again for even greater explicitness and transparency.   In Perl it
is straightforward to break the version number into components and
do arithmetic on them, so that’s what we do.  In shell that would be
tedious, so we are still using a case statement, but a lot of the
visual clutter has been factored out.

Also make the testsuite log the version of libtoolize, intltoolize,
gtkdocize, and autopoint, and whether it thinks autopoint supports
AM_GNU_GETTEXT without a version requirement.

Also add tests for autoreconf running autopoint under several
different conditions, and of autoreconf being directed *not* to
run autopoint.