3.2 PATCH: Fully support parallel gnat1/gnatbind builds

[Robert Dewar]

> I'm not opposed to requiring 3.1 Ada to compile 3.2 Ada.  It would be
> nice if we could preserve bootstrap compatibility farther back, but if
> there are major gains to maintainability from raising the bar, fine.

Well certainly we are not going to make incompatibilities for the sake
of making incompatibilties. But there are always cases in every release
where we preserve some kludge for maintainability.

A good example is unreferenced parameters. We require building with
warnings enabled and also the flag that makes warnings fatal. This means
that an unreferenced formal parameter is a fatal error. For compatibility
with old versions of GNAT, we use pragma Warnings (Off), but that's not
nearly so nice as using pragma Unreferenced (entity-name) which much more
clearly expresses the requirement, and avoids masking any other warnings.
We could not use pragma Unreferenced in 3.1, because it was not implemented
in earlier versions of GNAT, but we should at least consider this cleanup
for 3.2. We most certainly will do this cleanup internally at ACT for the
next version of GNAT Pro, since keeping the sources in maximally maintainable
shape is certainly an internal requirement we have at ACT.