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.