Criteria for a warning to be in -Wall? (was: Re: a warning to implement)

[Joe Buck]

Tim writes:
> Let's discuss what the conditions should be for a warning to be in
> -Wall, and then document it.  If that ends up meaning -Wnested-externs
> should be added, so be it.

Many projects and development teams, both free and non-free, require
clean compiles with -Wall.  What this means is that if we put an option
in -Wall, we are in a sense setting policy, the policy being that we
are asserting rather strongly that "you shouldn't do that".