Nix / Guix compatibility

CFFI fails to find libraries on Nix / Guix. This is (probably) because those two distributions don't follow the traditional filesystem hierarchy /lib, /usr/lib, etc.

For instance, trying to load sqlite-lib yields the following backtrace:

Nix doesn't give you a global sqlite3 version anyway… You can change $LD_LIBRARY_PATH or change CFFI's library search path to point to the version from Nixpkgs channel or checkout you prefer.