setup.py: use pkg-config for libffi include dir
Created originally on Bitbucket by scottt (Scott Tsai)
Was already merged in Bitbucket before import, marked as merged by the import user
Some Linux distros don't install ffi.h in the default header search path. This patch tries to use pkg-config(1) to find the correct include dir and does nothing if either pkg-config or libffi.pc isn't available. Distros that need this include Fedora, Red Hat and apparently Archlinux. Should fix cffi bitbucket issue#2.
Tested on Fedora 17, Ubuntu 10.04 and Ubuntu 12.04.