bug#25987: 25.2; support gcc fixit notes

[Eli Zaretskii]

> From: Andrea Corallo <akrl@sdf.org>
> Cc: David Malcolm <dmalcolm@redhat.com>,  25987@debbugs.gnu.org
> Date: Thu, 15 Oct 2020 14:23:05 +0000
> 
> > I'm not sure how a separate file comes into this.  Aren't we talking
> > about the "normal" GCC diagnostic output, just augmented by hints?
> > That has the advantage that it is also human-readable, and could help
> > the user make changes other than accepting the hints.
> 
> I explained this in my last mail on this thread, to make it short using
> stderr can't work reliably for parallel builds.

That's not a problem the feature discussed here can fix: it happens
with any parallel build run by "M-x compile".  The solution to that is
elsewhere (e.g., in using the GNU Make's command-line switches which
control parallelsim).