3.2 PATCH: Fully support parallel gnat1/gnatbind builds

[Joe Buck]

> 
> > Maybe the alternative is to be able to decide at configure time whether
> > to turn on -gnatg or not.  That way, users of older compilers could still
> > build, but they would see unrecognized pragmas warnings.
> 
> -gnatg is required for building the compiler.

Too bad: it appears that two theoretically independent notions were
combined: treating warnings as errors, and enabling maintainer mode.