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

[Lars Ingebrigtsen <larsi@gnus.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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.

[...]

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

I think we're saying the same thing.  😀

> 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.

There really are no rules for loaddefs.el -- Emacs itself (via
loaddefs-gen) figures out whether loaddefs.el needs updating.  I.e.,
Make knows nothing about the dependencies loaddefs.el really has.

We could make loaddefs.el depend on all .el files, but then we'd rebuild
the Emacs executable every time we change an .el file, and we don't want
to do that, either.