Add CI steps for `--enable-bundled-libffi`

[tenderlove]

@hsbt reported an issue with --enable-bundled-libffi here.

I added a build that uses this flag, and that build is failing. I was able to reproduce @hsbt's issue. I think it's caused by libffi installed with homebrew and using the --enable-bundled-libffi flag. My guess is that building the bundled libffi is failing, but the gem still compiles with the bundled ffi headers but links against the homebrew libffi.

I'm not sure how to fix yet, but I wanted to send this PR to show the issue.