Updated non-optimized patch.  The only real change is that I've moved 
some of the configuration stuff around (so not worth re-benchmarking 
this one);  I hope that I've now got the division of labour correct:

- configure script simply parses the --enable-big-digits option and sets
  PYLONG_DIGIT_SIZE in pyconfig.h to 15 or 30, or leaves it undefined
  if that option wasn't given to configure

- PC/pyconfig.h doesn't define PYLONG_DIGIT_SIZE

- pyport.h chooses a suitable value for PYLONG_DIGIT_SIZE if it's not
  already defined

- Include/longintrepr.h just follows orders: it expects 
PYLONG_DIGIT_SIZE
  to be defined already, and complains if PYLONG_DIGIT_SIZE=30 but the
  necessary integer types aren't available.

Thanks for all the benchmarking.

I'd probably better check on python-dev before pushing this in, since 
it's a new feature.  I hope no-one wants a PEP. :-)