Create a libaquaero library targets in the Makefile.

Creates a shared .a and .so files under the lib/ directory.
libaquaero: Make local functions static.

This prevents those symbols, not exported in the libaquaero5.h header,
to conflict with other programs.
libaquaero: Prevent memory leak when using default device.

The |full_path| path is constructed while scanning for the right
USB device but is never free'd.