Use library path logic built into FFI.
Use library path logic built into FFI.

The hardcoded library path lookup needed some fixing before. And it
still fails on a Ubuntu multiarch setup with both i368 and amd64 libcups
packages.

This reverts the library path lookup introduced in #3835be7a3e while
maintaining the possibility providing a CUPS_LIB using the environment.

nehresma/cupsffi