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I'd make unicode_filenames() a macro that expands to 0 on
platforms without this wart. I'd also test for wfunc!=NULL
before calling unicode_filenames().

There's a lot of hairy code here. Are you sure that there
are test cases in the test suite that exercise all of it?

Aren't there some #ifdefs missing? posix_[12]str have code
that's only relevant for Windows but isn't #ifdef'ed out
like it is elsewhere.

There should probably be a separate #define in pyport.h to
test for this that's equivalent to defined(MS_WINDOWS) &&
!defined(Py_UNICODE_WIDE), so this can be uniformly tested
to see whether this code is necessary.