devel: help in comparing Makefile.in from different commits

devel: help in comparing Makefile.in from different commits

Now that the generated Makefile.in, configure and aclocal.m4 files
are no longer committed in Automake's git repository, a simple
"git diff" or "git log" no longer shows if and how a change in
Automake results in changes to the Makefile.in files and/or
configure script of its own build system.  Still, the ability to
peek so easily at such differences has proved itself quite useful
in the past, often revealing inconsistencies and blunders, and
sometimes even bugs; so it would be a pity to lose that altogether.

With this change, we add a new maintainer recipe that re-introduces
much of that capability, by generating and comparing on the fly the
Makefile.in, configure and aclocal.m4 derived from two arbitrary
commits of the Automake repository.

* Makefile.am (autodiffs, compare-autodiffs): New phony targets.