Dependency tracking

[Akim Demaille]

Hi all!

Bison's backends are implemented in (GNU) M4.  Bison is shipped
with default M4 files (called skeletons), which m4_include several
bits.  Users may also write their own skeletons.

I regularly have the dependencies wrong in Bison's Makefile, and
some tests are not re-run when they should.  Fine tuning the
dependencies is tedious and error-prone, so in general I use a
superset of the dependencies, and some tests are run more than
needed.  Not a big deal, but since M4 knows what files are actually
used (bison itself does not know what M4 files are used, it knows
only about the top-level M4 file, but then m4 does what it wants),
it would make sense for it to tell us.

I made a quick search on this, and found that such a feature was
already designed (I found this:
https://lists.gnu.org/archive/html/m4-patches/2010-11/msg00001.html),
and actually contributed
(https://lists.gnu.org/archive/html/m4-patches/2011-02/msg00003.html).

Is there a reason for this work not to be part of M4?  I cannot
see any further reference after 2011-02 (but maybe I failed to
look for the right words, namazu does not seem very reliable).
I can see older messages, including interactions with Eric, but
nothing more recent.

Cheers!