FWIW, even with this patch and --without-threads Python does *not* build successfully on Cygwin64 (which is all I'm really interested in personally), though it does succeed on 32-bit Cygwin. This has two related reasons:
1) The build of Python's bundled libffi does not work on 64-bit Cygwin for at least a few reasons that I won't go into here. There doesn't seem to be a ticket yet for that so I'll open one.
2) Using --with-system-ffi doesn't work for reasons I talked about in #2445, and also discussed a bit in #1706863