Don't use conditional expressions.

Conditional expressions were introduced in 2.5. Officially that's the
earliest version we support because that's when ctypes was introduced,
but Plan 9 is still on 2.2 *and* it's one of the few platforms that don't
need ctypes, so.
Finally tested on Plan 9.

The Python from http://plan9.bell-labs.com/plan9/addons.html sets
sys.platform to 'unknown', which isn't particularly helpful. It will find
it anyway, obviously.