3.2 PATCH: Fully support parallel gnat1/gnatbind builds

[Robert Dewar]

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

That's because we wanted to make it IMPOSSIBLE for people to introduce
code that generated warnings. We find this a very useful discipline, and
not one that we would discard lightly just to solve one bootstrap path
problem that

a) can easily enough be solved by the reasonable requirement of using 3.1
to compile 3.2

b) is not that critical anyway, pragma Warnings (Off) is not terrible

But we do regard it as critical that no one be able to introduce a patch
that generates a warning. If you don't do that, it is all too easy for
such code to creep in, and no one has the energy to fix it up.