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: Wed, 03 Mar 2021 19:50:05 +0000
> 
> > AFAIU, when I make a change in Emacs C sources and rebuild Emacs, the
> > netive-compiled files will be put in a new directory under eln-cache,
> > right?
> 
> Essentially only if you add a primitive function.

Ah, okay, that's better.

> > Suppose I later would later like to remove stale binaries --
> > how do I know which eln-cache subdirectories I can remove at that
> > time?
> 
> ATM I tipically just remove all but the least recent one.  But another
> smarter technique might be looking at the subfolder name in the build
> tree you are interested in inside the 'native-lisp' directory, this is
> the same subfolder name that's used inside 'eln-cache'.

OK, but the name of the subfolder doesn't include the full version
number, I see 28.0.50-XXXXX, whereas the Emacs binaries are 28.0.50.1,
28.0.50.2, etc.

> Thinking about from Emacs one can find it simply inspecting the
> `comp-abi-hash' variable.

OK, so we can ask the binary itself which subdirectory it needs,
thanks.