Make the Interpreter use libffi if it's available. Patch from Alexei Svitkine!

This requires a rebuild of 'configure' itself. I will be committing that next, but
built with the wrong version of autoconf. Somebody who has the right one, please update
it.

As a side-note, because of the way autoconf works, all built tools will link against
libffi, not just lli. If you know how to fix this, please let me know ...

llvm-svn: 62553