Enable legacy multicast in Constants.

node/Constants.hpp

@@ -443,4 +443,9 @@
  */
 #define ZT_TEST_NETWORK_ID 0xffffffffffffffffULL
 
+/**
+ * Enable support for legacy <1.0.0 multicast peers (P5)
+ */
+#define ZT_SUPPORT_LEGACY_MULTICAST 1
+
 #endif