Add -enable-threads=gnat

[Arnaud Charlet]

> I don't understand the issues well enough to do that, so I cc'ed this to
> Arno, who does.  We may also want to consider having this automatically
> turned on when configuring Ada.

I'd add the following:

<<
@item gnat
Ada tasking support. This setting will by default be equivalent to
@samp{single}. Used in conjunction with the Ada run time, it will
use the same thread primitives.
This option is necessary when using Ada and the back end exception handling.
>>

To the table describing the --enable-threads switch.

Arno