Integrating libada branch

[Arnaud Charlet]

Nathanael,

I am trying to integrate the libada branch (to address
PR ada/6637, PR ada/5911), but I noticed a problem with
the current files: the directory libada is unconditionally configured
and maked, even with you use e.g. configure --enable-languages=c

I can't find what's missing, so I'd appreciate if you (or anyone else)
could give me some help.

Thanks in advance,

Arno