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I'm astonished how flexible the build-system is :-), once
you leave Windows.

setup.py needs a change, though, so that the _ctypes/libffi
configure script is not run when not needed.

One possible advange of Modules/Setup.local is that one can
define additional compilation flags for ctypes - for
example, if ctypes would support it, to enable/disable
certain features (varargs function calls, for example).