Speed up genattrtab

[Ralf Wildenhues]

* Mark Mitchell wrote on Tue, Jun 15, 2010 at 08:39:47PM CEST:
> But if some easy Makefile hackery would help some developers,
> I'd still argue we should do it.

IIRC, this hackery has already been done sometime last year or
so, with reordering of objects and the order-only dependencies.
Unless the build has changed a lot (haven't checked), it is
simply not possible to improve in a generally useful way
without changing GNU make to allow for more fine-tuning.

It is typically helpful to not increase N in -jN too much over
the number of available cores.

Cheers,
Ralf