bug#53891: 29.0.50; native comp eln cache

[Eli Zaretskii <eliz@gnu.org>]

> From: Colin McLear <mclear@fastmail.com>
> Date: Tue, 08 Feb 2022 23:10:24 -0500
> 
> On recent version from emacs-master I've noticed that I can no longer
> reliably set the eln-cache. I have the following in my early-init.el:
> 
> ```
> ;; Dir for eln-cache
> (when (boundp 'native-comp-eln-load-path)
> (setcar native-comp-eln-load-path
> (expand-file-name (convert-standard-filename ".local/temp/cache/eln-cache/")
> user-emacs-directory)))
> 
> ;; Silence nativecomp warnings popping up
> (setq native-comp-async-report-warnings-errors nil)
> 
> ;; Settings
> (setq native-comp-speed 2
> native-comp-deferred-compilation t
> package-native-compile t)
> ```
> But this isn't reliably setting the cache and I've had to rever to an
> earlier commit where things worked (as reported in the versio info).

This was never supported.  That it used to work for you was just luck.

Since it sounds like people want to divert the eln-cache directory to
custom places, something we didn't envision, we should support it
correctly: by making the "eln-cache" directory name customizable
instead of hard-coded.

I will work on adding such a feature.  Please stay tuned.