After an hour of fiddling with pre-requisites, I was able to run the libffi testsuite (for the Sun C build).  The results weren't ideal:

                === libffi Summary ===

# of expected passes            1528
# of unexpected failures        118
# of unsupported tests          55

And for the gcc 4.6 build:

                === libffi Summary ===

# of expected passes            1659
# of unsupported tests          55

I'm giving up.  (For now.)

Hopefully I'll be able to rope the libffi folk onto Snakebite in the future.