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

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Thu, 29 Aug 2019 14:30:54 -0400
> Cc: ferkiwi@gmail.com, 583@debbugs.gnu.org
> 
> rm -rf /tmp/foo
> mkdir -p /tmp/foo/.config/emacs
> HOME=/tmp/foo emacs
> 
> Now:
> user-init-file         -> /tmp/foo/.config/emacs/init
> user-emacs-directory   -> ~/.emacs.d/
> 
> I guess user-emacs-directory is being set at build time, not run time.

It's a defconst in subr.el, and I believe one of my review comments
for the initial version of the patch was that I didn't think using
defconst for this was correct.