[Patch, Ada, Darwin] Fix bootstrap issue on Darwin8.

[Arnaud Charlet]

> Darwin8 does not have _SC_NPROCESSORS_ONLN defined.

Is Darwin8 still active/supported?

> OK for trunk & 4.6?

Not as submitted, you haven't provided a suitable changelog for review.

Also, when you repost your patch, can you please send a single version of
the patch (either inline or as an attachment)? TIA.

Arno