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 13:56:24 +0000
> 
> > Why can't we have a rule in the Makefile conditioned by
> > HAVE_NATIVE_COMP?
> 
> We certainly can, the difficult part is to generate the rule as the .eln
> filename is known only by the Emacs binary.  I'm probably missing
> something.

Oh, you mean because of the ABI hash?  Yes, that'd preclude using Make
to decide when a .eln file needs to be regenerated.

> > Each time we add another Emacs invocation in the build process, we
> > make the goal of supporting cross-build farther.
> 
> Point taken.
> 
> [ To be considered also that as of today libgccjit is not meant to work
> for cross compilation. ]

Then perhaps we could invoke Emacs only in order to detect when the
ABI has changed.  Because when that happens, we need to regenerate all
the preloaded *.eln files anyway, so there's no need to test
individual files.  Right?