cpp(lib) vs. pedantic warnings vs. -Werror.

[Gabriel Dos_Reis]

| I'd like to have a single function, say cpp_diagnostic, that takes a
| severity flag, with slightly finer control on diagnostic types than we
| have now.  It should eventually all be integrated with the front ends,
| too, but one step at a time.

A while ago, Zack made the suggestion of using the diagnostic
machinery in gcc/diagnostic.* for cpp instead of replicating codes.
I don't know how far that idea went.  
However, I'm in the process of extending the diagnostic machinery in
order to permit finer diagnostics like `Notes:' (see a patch from
Geoff). 

-- Gaby