Ada and bad configury architecture.

[Nathanael Nerode]

Arno commented in a bug report:
> Also, having the TOOLS_TARGET_PAIRS being now set in configure.ac is
> more painful to maintain than having the list in Makefile.in, what about
> reverting this change ? It would make life simpler when adding support
> for a new target (no need to worry about rerunning configure, configure
> version, configure syntax errors, etc...)
This can't be reverted without abandoning all efforts to make
gnattools/gnatlib build like other target tools/libraries.

Actually, I was going to try to convince y'all to allow the *configury*
to be put in the *configure* files.  All of it.  The current scheme of
stuffing the configury in the Makefile, although I know the Ada
maintainers like it, is just trouble, and is fundamentally the source of
most or all of the endless Ada cross-build problems.

We could certainly create a file much like config.gcc in gcc, so that
'autoconf' doesn't need to be rerun, but 'configure' *should* need to be
rerun to change configuration.