message: mavlink_commands: attributes not ok for GCC 4.8
  The comment FALLTHRU is what both seem to understand, GCC 4.8 and 7.
diff: @@ -157,11 +157,11 @@ void MavlinkCommands::receive_command_ack(mavlink_message_t message)
  case MAV_RESULT_DENIED:
  Debug() << "command denied (" << work.mavlink_command << ").";
  - [[fallthrough]]; // no break
  + // FALLTHRU
  case MAV_RESULT_UNSUPPORTED:
  Debug() << "command unsupported (" << work.mavlink_command << ").";
  - [[fallthrough]]; // no break
  + // FALLTHRU
  case MAV_RESULT_FAILED:
  Debug() << "command failed (" << work.mavlink_command << ").";
  