Last (hopefully?) step before getting Next up and running: add support for CFFI.

I've re-used your CFFI package and they seem to work.  Now if I want to package
cl-sqlite, it fails because sqlite.so cannot be found.  Indeed, the patch to
/gnu/store/...-sqlite.../lib must be added to the
cffi:*foreign-library-directories* global variable before compiling.

If I do that from quicklisp, it works.  Now I guess we need to tweak
asdf-build-system so that packages can specify which paths to add to that global
variable.

Makes sense?

I've given it a quick shot (patch attached) but then every ASDF build fails with

--8<---------------cut here---------------start------------->8---
ice-9/boot-9.scm:222:17: In procedure map1:
Syntax error:
unknown location: unexpected syntax in form ()
--8<---------------cut here---------------end--------------->8---

I'm still lacking good knowledge of the asdf build system.

I'll give it a closer look tomorrow.