libpri is enabled by default and breaks the emerge when freetdm is disabled (also the default)

[stefantalpalaru]

The emerge fails with a complain that libpri was not found. A workaround is to put FREETDM_MODULES="" in make.conf but a better fix would be to put a higher level check for freeswitch_modules_freetdm in src_configure() wrapping the freetdm_modules_* checks