[Ada] Patch to fix bug reporting instructions (3.1 branch)

[Joe Buck]

I wrote:
> > I think that it suffices to provide accurate installation instructions.
> > It's too hard to produce software that detects and behaves correctly in
> > any possible situation.  That is, if someone follows the instructions they
> > get a working compiler.

Joseph Myers wrote:
> As far as I'm concerned, the installation instructions are accurate but
> incomplete; they document the use of gnatgcc, and different C and Ada
> compilers, but do not state the required versions.  I've applied the
> following patch (mainline and 3.1 branch) to document what I understand
> the GNAT version requirements to be, and the lack of configure sanity
> checks.

That looks fine. [ patch deleted ]

> I would support the addition to the documentation of a URL where bootstrap
> compiler binaries that are tested to work can be downloaded.

Janis has been maintaining the build status page; that seems to be an
appropriate way to track Ada builds as well.

I'm sure that when we ship, we will find a few unpleasant surprises.
These might require fixes in 3.1.1.