bug#46256: [feature/native-comp] AOT eln files ignored if run from build tree

[Eli Zaretskii]

> From: Andrea Corallo <akrl@sdf.org>
> Cc: 46256@debbugs.gnu.org
> Date: Fri, 05 Mar 2021 09:32:35 +0000
> 
> The trouble of using make for building such a system is that make is not
> aware of the .eln filename, so it should be necessary to ask the Emacs
> binary about that to create dynamically the precise (multiple target)
> rule.  Not very practical IMO...

Why can't we have a rule in the Makefile conditioned by
HAVE_NATIVE_COMP?

> Another option would be to invoke Emacs only once passing to it the list
> of the .el files to be compiled and the parallelism requested and have
> Emacs do the job.  I think this might be easier and we have in the
> codebase already the all that's needed for that.  The downside is that
> we'd drift away from how the vanilla build is working.

Each time we add another Emacs invocation in the build process, we
make the goal of supporting cross-build farther.