bug#31737: 26.1.50; Gnus does not use mailutils movemail

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: m43cap@yandex.com,  31737@debbugs.gnu.org
> Date: Fri, 08 Jun 2018 18:01:14 +0200
> 
> > Sorry, I don't think I follow: mail-source-movemail-progam is a
> > variable in Gnus, and its default is nil.
> 
> Yes, and in that case Gnus looks for movemail in 'exec-directory'
> only, where emacs no longer installs it if it found Mailutils
> movemail at configure time.

But then the fix should be where mail-source-movemail-progam is used,
not in its default value, right?  Gnus should use executable-find to
look for the program named by mail-source-movemail-progam.

> >> [1]  Eli, 26.2? This is an emacs-26 regression
> >
> > What exactly was the regression, and which change do you propose for
> > the branch?
> 
> Gnus can no longer use movemail, even though at configure time it was
> found.

And why is that an emacs-26 regression?  the value of that variable
hasn't changed since 2004, AFAICS.

> The simplest fix is below, but it will cause people who currently have
> Mailutils movemail installed, but an emacs built to not use it, to
> start using it, so it makes me uncomfortable. Perhaps gnus should
> check in 'exec-directory' first if mail-source-movemail-program is
> nil, and then fall back to 'executable-find'?

I see no need for any special handling by Emacs here: the user could
customize the value to point to "our" movemail as an absolute file
name, in which case executable-find will pick it up.  We just need to
change Gnus to use executable-find where the variable is used.