Hmm, Cygwin64 is what I built on (successfully).  _ctypes does not build, but that's due to not having any libffi available at all: the bundled copy is no longer included in 3.7, and I didn't install a system copy.  I view ctypes support as a fairly low-priority item; we have much bigger issues to worry about first :).  If trying to build _ctypes actually crashes your build, I think '--without-system-ffi' might be enough to get it to error out cleanly.