Autotools updates and fix FFI detection

Our config.{guess,sub} haven't been updated in decades... Until
today. This set of changes also brings a fix for FFI detection so now
if your libffi has a pkg-config file installed we should be able to
find your headers.

* ffi-autoconf:
  Update configgmp.{guess,sub} from GMP v6.2.0
  Build Fix -- FFI auto-detection via pkg-config