The attached patch makes libffi compile on Tru64 5.1B.  Without it,
the compilation stops on this in src/alpha/osf.S:

#error "osf.S out of sync with ffi.h"

Since "make check" requires dejagnu but I don't have it, I tried
the "puts" example from http://www.atmark-techno.com/~yashi/libffi.html, 
and it worked.

(It would be good to have some trivial test suite that works
without dejagnu...)

Compiled with the ancient gcc 2.9 the system has.

The patch simply removes the check of FFI_TYPE_LAST being 14 or 
FFI_TYPE_POINTER.  Not necessarily the most correct thing to do,
but hey.

I have also access to an IRIX 6.5 box, but there the situation
seems to be more dire.  (gcc 3.3, FWIW.)