diff --git a/make/subtree.mk b/make/subtree.mk
index 2324c414..0817b2d2 100644
--- a/make/subtree.mk
+++ b/make/subtree.mk
@@ -25,6 +25,9 @@ SUBTREE_FILES               += $(shell for f in `git ls-files . 2>/dev/null || f
 INSTALLED_SUBTREE_DIRS       = $(addprefix $(ENV_PREFIX)$(SUBTREE_INSTALL_PREFIX)/,$(SUBTREE_DIRS))
 INSTALLED_SUBTREE_FILES      = $(addprefix $(ENV_PREFIX)$(SUBTREE_INSTALL_PREFIX)/,$(SUBTREE_FILES))
 
+ifndef SUBTREE_MK_INCLUDED
+SUBTREE_MK_INCLUDED = true
+
 include $(JWBDIR)/make/defs.mk
 
 all:
@@ -37,3 +40,5 @@ $(INSTALLED_SUBTREE_DIRS):
 
 $(ENV_PREFIX)$(SUBTREE_INSTALL_PREFIX)/%: % | $(INSTALLED_SUBTREE_DIRS)
 	$(INSTALL) -p -o $(SUBTREE_OWNER) -g $(SUBTREE_GROUP) -m $(SUBTREE_MODE) $< $@
+
+endif # SUBTREE_MK_INCLUDED