bug#50852: [PATCH] Fix search of the look program.

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 27 Sep 2021 16:56:14 -0700
> Cc: 50852@debbugs.gnu.org
> 
> André A. Gomes <andremegafone@gmail.com> writes:
> 
> > Here's a more robust way to handle the existence of the look program by
> > ispell.  GNU Guix users will be happy.
> 
> LGTM.  I do wonder if this shouldn't be done in many more places...

Careful: that will subtly change the behavior.  For starters,
executable-find looks in exec-path, which includes Emacs-specific
directories.  It also has its own ideas about which files are
executable and whether to try some extensions.  And finally,
executable-find could return nil, whereas the original value was never
nil, which could cause some code to signal an error, and requires the
user to redefine the value when a program was originally absent, but
is then installed without restarting Emacs.

I'm not saying these differences are for the worse, but they need to
be carefully considered when making such "innocent" changes, as they
could introduce subtle bugs and misfeatures, at least potentially.

In this case, why not simply have the value as "look" with no leading
directories?  If the program is installed, it will be found when it's
invoked, and if it isn't installed, the user gets an error message at
that time.