build: make VLAN code compatible with older kernels

Apparently VLAN_FLAG_LOOSE_BINDING isn't in older kernels like 2.6.32.

configure.ac

@@ -261,6 +261,28 @@ if test "$ac_have_nl80211" = no; then
 	AC_MSG_ERROR(Linux kernel development header linux/nl80211.h not installed or not functional)
 fi
 
+dnl
+dnl Default to using WEXT but allow it to be disabled
+dnl
+AC_MSG_CHECKING([Linux kernel VLAN_FLAG_LOOSE_BINDING enum value])
+AC_COMPILE_IFELSE(
+	[AC_LANG_PROGRAM(
+              [[#ifndef __user
+                #define __user
+                #endif
+                #include <sys/types.h>
+                #include <linux/types.h>
+                #include <linux/if_vlan.h>]],
+              [[unsigned int a = VLAN_FLAG_LOOSE_BINDING;]])],
+	[ac_have_vlan_flag_loose_binding=yes],
+	[ac_have_vlan_flag_loose_binding=no])
+AC_MSG_RESULT($ac_have_vlan_flag_loose_binding)
+if test "$ac_have_vlan_flag_loose_binding" = yes; then
+	AC_DEFINE(HAVE_VLAN_FLAG_LOOSE_BINDING, 1, [Define if you have VLAN_FLAG_LOOSE_BINDING])
+else
+	AC_DEFINE(HAVE_VLAN_FLAG_LOOSE_BINDING, 0, [Define if you have VLAN_FLAG_LOOSE_BINDING])
+fi
+
 dnl
 dnl Checks for libm - needed for pow()
 dnl

src/nm-system.c

@@ -65,6 +65,11 @@
 #include <netlink/route/link/vlan.h>
 #endif
 
+#if !HAVE_VLAN_FLAG_LOOSE_BINDING
+/* Older kernels don't have this flag */
+#define VLAN_FLAG_LOOSE_BINDING 0x04
+#endif
+
 static void nm_system_device_set_priority (int ifindex,
                                            NMIP4Config *config,
                                            int priority);