Fix detection of libffi on MacOS (or other OS with dodgy system libffi).  Based on patch from Jeremy Hinegardner <jeremy@hinegardner.org>
Fix detection of libffi on MacOS (or other OS with dodgy system libffi).  Based on patch from Jeremy Hinegardner <jeremy@hinegardner.org>

majioa/ffi,mvz/ffi,tduehr/ffi,MikaelSmith/ffi,majioa/ffi,ffi/ffi,ferventcoder/ffi,tduehr/ffi,yghannam/ffi,sparkchaser/ffi,sparkchaser/ffi,majioa/ffi,yghannam/ffi,mvz/ffi,yghannam/ffi,mvz/ffi,ferventcoder/ffi,MikaelSmith/ffi,MikaelSmith/ffi,mvz/ffi,ferventcoder/ffi,ffi/ffi,tduehr/ffi,ffi/ffi,yghannam/ffi,majioa/ffi,tduehr/ffi,ferventcoder/ffi,sparkchaser/ffi,sparkchaser/ffi,yghannam/ffi,MikaelSmith/ffi