Integrating libada branch

[Arnaud Charlet]

> Sure...

Thanks for the patch, that's very helpful.

I guess we should actually revert the default.
That is to say, enable libada if Ada is enabled, and disable it
if --disable-libada is specified.
This way, people won't forget to specify it, since
the issue currently is that people "forget" to do a make -C gnatlib_and_tools,
so having libada disabled by default wouldn't really improve the situation.

Arno