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I tried your patch on an AMD64 ubuntu 5.10, after I
installed the 4.0.1-4ubuntu9 libffi package.  It crashes in
ctypes/test/test_python_api.py, test_PyOS_snprintf.  IIRC,
libffi did not support varargs functions, but Andreas Degert
added support for them (for x86_64), and it works fine.

Ok, new features in libffi might not be the norm, but I see
no way how I can check whether a system-installed libffi
supports this feature or not.

Do *you* see a solution for that?

For the actual changes to setup.py, if the patch really is a
good idea:  Since I don't even know what a convience library
is I'll trust you fully that your patch does the right thing.