Let's fix how warnings are specified

[Mark H Weaver]

Hello all,

At present, compile-time warnings can only be enabled, not disabled, and
there is no way to enable all warnings.  This means that the set of
warnings has to be hard-coded into every build system.  This is terrible
because it means that every time we add a new warning type, users won't
benefit from it unless they update their Makefiles.

We need a way for users to ask for a reasonable default set of warnings
(which should be almost all of them), possibly with some warnings
explicitly enabled or disabled.  We should also have a way to enable all
warnings.

It would also be nice if this could be done in such a way that external
build systems can do this without breaking compatibility with Guile
2.0.[0-5].

I'm looking for ideas of how to adjust our 'compile' and 'guild compile'
interfaces to accomplish these objectives.

Here's a preliminary proposal:

* Add new pseudo-warning types 'all' and 'default'.
* For each specific warning type 'FOO', we add 'no-FOO'.
* These new pseudo-warning types would be honored by both
  'compile' and 'guild compile'.

What do you think?

    Mark