oh yay, thanks.  it looks like you did approximately what i had started
working on testing a while back but have gone much further and added
autoconf magic to try and determine when which size should be used.  good.

(i haven't reviewed your autoconf stuff yet)

As for marhsalled data and pyc compatibility, yes that is important to
consider.

We should probably base the decision on which digit size to use
internally on benchmarks, not just if the platform can support 64bit
ints.  Many archs support 64bit numbers as a native C type but require
multiple instructions or are very slow when doing it.

(embedded arm, mips or ppc come to mind as obvious things to test that on)