python-cffi: enable out-of-tree builds

This is a simple hack to enable out-of-tree builds, a concern raised
by Tomi in id:m24kzjib9a.fsf@guru.guru-group.fi

This change at least enables "make check" to complete without error,
but I'm sure it could be improved.  I am not expert enough in
setuptools to know how.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Amended by db per id:87d06usa31.fsf@powell.devork.be