ctypes always uses the included libffi implementation
(which should be updated to current GCC HEAD). Pleae
add an option to build and link against a libffi, which
is installed on the system.

attached is a hack to just always build using the
installed libffi. I'm unsure how to find the correct
ffi implementation (maybe check for LIBFFI_H defined in
the ffi header?), and where to implement this check
(configure.ac --with-system-ffi=/usr?).