Set all in place (for Linux systems) to load automatically libgm (gpu_mod)...

Set all in place (for Linux systems) to load automatically libgm (gpu_mod) from pyfaust (whether it was installed from a pip package or a system package).

Wrapper pyfaust.enable_gpu_mod() calls C++ core to load the lib.
Symlink libgm into pyfaust when packaging a system package (we don't want to duplicate libgm for pyfaust and matfaust).