mostly backport from upstream https://github.com/open62541/open62541.git

--- qt-everywhere-src-6.9.3/qtopcua/src/3rdparty/open62541/open62541.c.orig
+++ qt-everywhere-src-6.9.3/qtopcua/src/3rdparty/open62541/open62541.c
@@ -123774,7 +123774,7 @@
 UA_StatusCode
 UA_EventLoopPOSIX_setReusable(UA_FD sockfd) {
     int enableReuseVal = 1;
-#ifndef UA_ARCHITECTURE_WIN32
+#ifdef SO_REUSEPORT
     int res = UA_setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR,
                             (const char*)&enableReuseVal, sizeof(enableReuseVal));
     res |= UA_setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT,
@@ -125228,7 +125228,7 @@
 } MultiCastType;
 
 typedef union {
-#ifdef UA_ARCHITECTURE_WIN32
+#if !defined(ip_mreqn)
     struct ip_mreq ipv4;
 #else
     struct ip_mreqn ipv4;
@@ -125384,10 +125384,12 @@
         return UA_STATUSCODE_BADINTERNALERROR;
 
     /* Write the interface index */
-    if(info->ai_family == AF_INET)
+    if(info->ai_family == AF_INET) {
+#if defined(ip_mreqn)
         req->ipv4.imr_ifindex = idx;
+#endif
 #if UA_IPV6
-    else /* if(info->ai_family == AF_INET6) */
+     } else /* if(info->ai_family == AF_INET6) */
         req->ipv6.ipv6mr_interface = idx;
 #endif
     return UA_STATUSCODE_GOOD;
@@ -125402,7 +125404,7 @@
     if(info->ai_family == AF_INET) {
         struct sockaddr_in *sin = (struct sockaddr_in *)info->ai_addr;
         req->ipv4.imr_multiaddr = sin->sin_addr;
-#ifdef UA_ARCHITECTURE_WIN32
+#if !defined(ip_mreqn)
         req->ipv4.imr_interface.s_addr = htonl(INADDR_ANY); /* default ANY */
 #else
         req->ipv4.imr_address.s_addr = htonl(INADDR_ANY); /* default ANY */