Re: clean up m4sh.m4, then optimize AS_PATH_WALK

[Eric Blake]

> > So this patch was not only not fixing a bug, but merely an optimization,
> > but also introducing a bug.
> >
> > Please leave the introduction of bugs to post 2.60 for now, unless you
> > also fix actual bugs.  Micro-optimization doesn't count for the latter.

Sorry about that - I guess I had not thought about the full consequences
of changing semantics of a core macro, especially this close to a release,
when my only goal was a faster configure file.

> 
> But we can do better!  We can optimize out _all_ extra forks for nonexistent
> shells!  I hope you will count this as a bug fix!  :-)

I'll leave that to Ralf or others to decide.

> 
> >     * lib/m4sugar/m4sh.m4 (_AS_PATH_WALK): Optimize nonexistent
> >     directories, unless optional third argument supplied.
> >     (AS_UNAME): Don't optimize PATH walk.
> 
> On a second thought, I shouldn't have accepted that patch.
> If any caller needs to skip nonexistent directories, he can do so by
> insering the command
>       test -d "$ac_dir" || continue
> to the beginning of the second parameter.

Fair enough.  Go ahead and revert my patch (but your proposed patch
is too small - you also need to revert my comment changes).

-- 
Eric Blake