bug#583: Use XDG basedir spec for configuration files?

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Fri, 30 Aug 2019 12:18:40 -0400
> Cc: ferkiwi@gmail.com, 583@debbugs.gnu.org
> 
> Although in hindsight an environment variable doesn't seem like the way
> to go.

Actually, I think it does, because otherwise the user-defined value
will be noticed too late, after defcustom's that depend on it were
already recomputed.