Re: monstro-patch: machinery to enable/disable warning/error categories

[Akim Demaille]

| Automake doesn't have "warnings", it can only print "errors"
| that cannot be muted and cause a non-0 exit status.  Several
| diagnosis have never been implemented because of this (e.g. we
| don't warn if the user override CFLAGS instead of AM_CFLAGS in
| it's Makefile, because reporting this as an `error' would hurt
| many users).
| 
| This patch adds support for warning categories (-Wmumble).

This is great!

| The new file lib/Automake/Channels.pm contains the machinery
| to handle messages channels.  The main interface is `msg'.

This is excellent too!  This will help merging this into Autoconf
too.  The more I see how Automake and Autoconf evolve, the more
seriously I consider merging.

| Once a channel is registered (let's say it's called 'unused'),
| messages can be emitted with `msg':
| 
|   msg 'unused', "$file:$line", "unused variable `$var'";
| 
| The channel 'unused' can be configured to be a warning, or an
| error (in which case it update the exit_status), it can be muted
| (think -Wno-unused), it can be told to filter duplicate messages
| (this simplify a few places in Automake where this was done
| manually).  See the embeded POD for the other options.

I've not read yet, but what I would most enjoy is:

1. support for $WARNINGS
2. -Wall, -Wnone etc. à la Autoconf
3. exporting the current warnings in $WARNINGS to pass them
   to subtools.

It is important not to reject wrong -Wfoo.  You may warn about them if
-Wwarnings ;)

Then, we should submit -W/--warnings to the GCS.