configure.ac: add lib path searches for Debian multiarch

This fixes #4, and I think is currently the most robust way of handling multiarch for Debian environments with autoconf builtins.  It extends the lib search path only if it detects the existence of multiarch folders matching the default or requested (via `--host` when running `./configure`) 'host' build variables for CPU and OS (examples: i386-linux-gnu, x86_64-linux-gnu).

This retains the user's capability of overriding where libs come from with the `--with-staticlib` option.