message: fix preprocessor cmd
diff: #define CPPHTTPLIB_RECV_FLAGS 0
  #endif
  -#if !defined MSG_NOSIGNAL
  -#define MSG_NOSIGNAL 0
  -#endif
  -
  -#ifndef CPPHTTPLIB_SEND_FLAGS
  +#ifndef MSG_NOSIGNAL
  +#define CPPHTTPLIB_SEND_FLAGS 0
  +#else
  #define CPPHTTPLIB_SEND_FLAGS MSG_NOSIGNAL
  #endif
  