Just a FYI, this is an encapsulation of a email thread from a month
ago or so -- if you're missing ffi.h when installing on 10.5.8, you
should

port install libffi

An even better non-workaround would be to change extconf.rb in the ffi
gem to actually build libffi (which is included in the ext/ffi_c/
libffi directory), and modify the include and link paths in the
Makefile accordingly if ffi.h isn't originally found.

Brian Moran
http://www.onehub.com