DR handling for C++

[Paolo Bonzini]

> The name pedwarn() has been a bit confusing (because it suggests a link to 
> -pedantic which isn't really there, the link being that -pedantic-errors 
> turns pedwarns from warnings to errors), but I don't have a good 
> suggestion for a less confusing name.

-pedantic-errors is itself not very well named... maybe -Wextra-errors 
or something like that would be more suited now that -W has become -Wextra.

Paolo