llvm3{3,4}: support --enable-ffi build option

For the LLVM builds, this option to ./configure depends on libffi and enables calling native functions from interpreted LLVM bitcode. See [LLVM's packaging docs](http://llvm.org/docs/Packaging.html#dependencies).

This is my first contribution to an "official" homebrew repo, so let me know if there are any style issues to correct.