Fix build on Snow Leopard

Ignore-this: fcccd6dc708eb6d4810ef31a6595e15a

Now build will be 32bit and linked against 10.5 SDK if using
Python 2.5. That will generate a 32bit binary compatible with
Leopard.

darcs-hash:20100506162502-56b4d-99f569f6652a787063565282ce5e50d5dbb1cd37
Lower cython version dependency

Ignore-this: 994c4aebc0cbe2b54e2fd59cc5db8507

darcs-hash:20100507155650-58252-8ec74d4f9ed4650293e5842802357d5af2d80afb