libffi / pkgconfig doesn't find libffi.dylib on mac in standard place

works when brew link libffi --force is done, but thats kinda hack

blocks idris current release from building out of the box, and the brew workaround is not a "robust" / good solution

Should be able to pick up the /usr/include and /usr/lib stuff (note those are gone in 10.9 though, cf various docs via apple and cabal's issue tracker )