[m-rev.] for review: implement Mantis 497

[Peter Wang]

On Sun, 05 Apr 2020 19:06:02 +1000 (AEST), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
>  
> > Can you add a NEWS entry as well?
> 
> What should that entry say? I don't see much point
> in telling people that they can now put as-yet-invalid
> warning names in disable_warnings scopes, since *users*
> have no way to make them valid later; only developers do.

I've added this:

* [Mantis bug #497]. The compiler will now generate a warning for an
  unrecognised warning name in a `disable_warnings` scope instead of
  reporting an error.

Peter