docs: re-introduce mention of two-args AM_INIT_AUTOMAKE invocation

docs: re-introduce mention of two-args AM_INIT_AUTOMAKE invocation

Albeit obsolescent and raising warnings in the 'obsolete' category,
that usage is still supported, and will need to be until Autoconf
improves its handling of configure-time-generated package version
numbers.  So it's better to explicitly document it again, stating
that it is obsoleted but still working (and why), rather then leaving
it as Yet Another Undocumented Feature (that will mysteriously and
suddenly break some random day in the future).

It's worth giving some background about how we ended up in the
situation that this patch fixes.

We had originally removed support for the long-deprecated two-args
AM_INIT_AUTOMAKE invocation (see commit v1.12-67-ge186355).  Before
that removal could land in a released Automake version, Bob Friesenhahn
made a quite compelling point that the two-args AM_INIT_AUTOMAKE
invocation could still be useful for modern, maintained packages like
GraphicsMagick, at least until Autoconf is fixed to offer better support
for "dynamic" package versions (see commit v1.12.2-245-g2abe183 for more
in-depth rationales and references).  However, in that commit we didn't
revert the removal of the *documentation* for this two-arguments
AM_INIT_AUTOMAKE invocation (and no rationale for not doing so was given
in the commit message).  Time to remedy that.

Indirectly suggested by Diego Elio Pattenò:
<http://blog.flameeyes.eu/2013/01/autotools-mythbuster-automake-pains>

* doc/automake.texi: Adjust.
* NEWS: Update.
* THANKS: Likewise.

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