[[fallthrough]] if C++17 only

libi2pd_client/SOCKS.cpp

@@ -433,7 +433,9 @@ namespace proxy
 						break;
 						case CMD_UDP:
 							if (m_socksv == SOCKS5) break;
+#if (__cplusplus >= 201703L) // C++ 17 or higher							
 							[[fallthrough]];
+#endif							
 						default:
 							LogPrint(eLogError, "SOCKS: invalid command: ", ((int)*sock_buff));
 							SocksRequestFailed(SOCKS5_GEN_FAIL);