Don't bother trying to work out which commands require cffi and which do not as 'use_system' is called for all of them

Apply some of the techniques from https://caremad.io/2014/11/distributing-a-cffi-project/ in the hope of making 'pip install' work reliably for pynacl

Relicensed with permission of David Fischer <djfische@gmail.com>.

Unfortunately, this only works if cffi is already installed