bug#57152: 29.0.50; Emacs executable isn't rebuilt when loaddefs.el is modified

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 57152@debbugs.gnu.org
> Date: Mon, 15 Aug 2022 12:07:15 +0200
> 
> I've tried various things, like making autoloads an order-only
> prerequisite for $(pdmp) (and various other targets), but the main
> problem seems to be that Make has already computed the
> timestamps/dependencies at this point, so even if we're running the
> update at the "right" time, we don't trigger the $(pdmp) rule.

That cannot be the reason, AFAIK.  Make begins by reading all the
rules and dependencies in the Makefile and producing a DAG for
rebuilding the targets; then it walks the DAG invoking commands as it
goes.  Thus, a target that is going to be rebuilt will automatically
trigger the rules of all the targets that depend on it, as long as the
dependencies are known to Make in full.

IOW, Make doesn't update the timestamps of files during the build, it
figures it all out in advance.

So I think the problem here is that the rules which trigger
regeneration of loaddefs.el are in lisp/Makefile, while the rules for
building $(pdmp) are in src/Makefile.

So maybe moving or duplicating the loaddefs.el rules in src/Makefile
will do the trick.