message: Check for MSG_NOSIGNAL instead of WINDOWS
  This resolves a build failure on macOS 10.15.
  Related:
diff: @@ -158,7 +158,7 @@ bool TcpConnection::send_message(const mavlink_message_t& message)
  // TODO: remove this assert again
  assert(buffer_len <= MAVLINK_MAX_PACKET_LEN);
  -#if defined(WINDOWS)
  +#if !defined(MSG_NOSIGNAL)
  auto flags = 0;
  #else
  auto flags = MSG_NOSIGNAL;
  