cffi does not admit the possibility of libffi.so.8

One fix is to add to line 34 ~quicklisp/dists/quicklisp/software/cffi_0.24.1/libffi/libffi.lisp

  (:unix (:or "libffi.so.8" "libffi.so.7" "libffi32.so.7" "libffi.so.6" "libffi32.so.6" "libffi.so.5" "libffi32.so.5"))

Reporting it here as I discovered it trying to get sketch to run on ArchLinux where the libffi is installed via pacman.

I am a lisp noob. Appreciate comments on a better fix than editing the source code of some quicklisp installed package.

Should be fixed once this this pull request to cffi makes into the quicklisp version cffi/cffi#191