>port installing libffi did the trick. That perhaps ought to have been
>obvious, but perhaps making it an explicitly checked precondition of
>the build script might help others.

libffi is assumed to be installed in the system ... and I'd suggest that when installing the gem reports this error:

>Types.h:74:17: error: ffi.h: No such file or directory

that it is an "explicitly checked precondition" ;-)