I'm unsure how to proceed with this.
Replacing the copy of libffi in the ctypes sources - I'm very afraid to
do that.  It has it's own configure system, written by someone else. 
Then it has several changes from various people for Python, which would
be overwritten.  All these on systems that I do not have direct access
to, and we do not have buildbots for all of them.

So, it looks like --with-system-ffi should be made the default (on
systems that have libffi installed?)

There are also test failures since I committed the c_longdouble patch
(on alpha debian, and S390 Debian, for example).  Maybe newer libffi
versions have a fix for that...

Last comment:  Experimental changes COULD be tested out in a branch, we
would only have to manually trigger the build from the buildbot web pages.