+option(ENABLE_IAX "Set to ON to build FlightGear with IAXClient/fgcom built-in (default)" ON)
+option(USE_DBUS "Set to ON to build FlightGear with DBus screensaver interaction (default on Linux)" ${USE_DBUS_DEFAULT})
+option(SYSTEM_SPEEX "Set to ON to build IAXClient with the system's speex and speexdsp library" ${SYSTEM_SPEEX_DEFAULT})
+option(SYSTEM_GSM "Set to ON to build IAXClient with the system's GSM library" ${SYSTEM_GSM_DEFAULT})
+option(SYSTEM_FLITE "Set to ON to build Flightgear with the system's Flite library" ${SYSTEM_FLITE_DEFAULT})
+option(SYSTEM_HTS_ENGINE "Set to ON to build Flightgear with the system's HTS Engine library" ${SYSTEM_HTS_ENGINE_DEFAULT})