I've defined this macro just to test libev/boost::asio cases depending on it.

Previously this example is forced to use `boost::asio::io_service` (see `const bool use_boost = true`), but it hanged forever and there was no other way to stop it except killing it, because SIGTERM was caught by libev signal handler, which (surprisingly) stopped libev event loop.

If you want, I'll return back `const bool use_boost` variable, but I think it looks strange in this context.