Make utp_config.h usable with MSVC (#warning directive is not standard)

third-party/utp_config.h

@@ -5,7 +5,12 @@
 // This should return the global number of bytes sent, used for determining dynamic
 // packet size based on rate
 
+#ifdef _MSC_VER
+#pragma warning()
+#pragma message(implement this in libtransmission)
+#else
 #warning implement this in libtransmission
+#endif
 uint64 UTP_GetGlobalUTPBytesSent(const struct sockaddr *remote, socklen_t remotelen) { return 0; }
 
 enum bandwidth_type_t {