[gtkada] improvements to the GtkAda build system

[Arnaud Charlet]

> I was going to bring this up as a separate topic.  Since it's now known
> that GNAT FSF can't compile gtkada, why not apply these patches to the
> code base?

Because these patches are wrong: they will generate a compilation error
in more recent versions of GNAT, so they should be considered as a work around.

> Do people really want a patch to be required for FSF GNAT to compile GTKAda?

Note that it's only testgtk (a test program) which is causing trouble, not
the GtkAda library per se.

I've looked at the code, and made a better change to remove the need for a
recent GNAT, hopefully, so making this patch no longer necessary.

Arno