fix unix detection in wrong place

src/net.c

@@ -267,7 +267,7 @@ void mg_mgr_init(struct mg_mgr *mgr) {
   // clang-format on
 #elif MG_ENABLE_FREERTOS_TCP
   mgr->ss = FreeRTOS_CreateSocketSet();
-#elif defined(__unix) || defined(__unix__) || defined(__APPLE__)
+#elif MG_ARCH == MG_ARCH_UNIX
   // Ignore SIGPIPE signal, so if client cancels the request, it
   // won't kill the whole process.
   signal(SIGPIPE, SIG_IGN);