Add make option ZT_ENABLE_CLUSTER and disable it in default Linux build.

make-linux.mk

@@ -29,7 +29,7 @@ endif
 UNAME_M=$(shell $(CC) -dumpmachine | cut -d '-' -f 1)
 
 INCLUDES=
-DEFS=-DZT_ENABLE_CLUSTER
+DEFS=
 LDLIBS?=
 
 include objects.mk
@@ -70,6 +70,11 @@ ifeq ($(ZT_ENABLE_NETWORK_CONTROLLER),1)
         OBJS+=controller/SqliteNetworkController.o 
 endif
 
+# Build with ZT_ENABLE_CLUSTER=1 to build with cluster support
+ifeq ($(ZT_ENABLE_CLUSTER),1)
+	DEFS+=-DZT_ENABLE_CLUSTER
+endif
+
 # "make debug" is a shortcut for this
 ifeq ($(ZT_DEBUG),1)
 	DEFS+=-DZT_TRACE