[Ada] Reenable parallel build

[Eric Botcazou]

> We could (although moving is not the right solution for users of
> --disable-libada),

I doubt there are many users of --disable-libada and we can document that the 
gnattools or gnatlib_and_tools targets are not parallel-safe and thus need to 
be invoked with -j1 explicitly.

> but as I said, there may also be troubles with parallel builds of gnatlib
> and gnattools (since gnattools needs to be built after gnatlib) in some
> cases. 

Can't this ordering constraint be expressed in the Makefiles?

-- 
Eric Botcazou