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.