[warning control] new -W[no-]error-foo option

[DJ Delorie]

> For this you really need every test to assert whether it is a warning or 
> an error, using
> 
>   dg-warning "warning: ..."
> 
> or
> 
>   dg-error "error: ..."

This doesn't seem to fully check the case with -Werror, because we
print warnings (i.e. use dg-warning "warning: ...") but we need to
verify that the compiler didn't in fact produce an output file.  Is
there some quick way to check the exit code of gcc?