diff --git a/rules.mk b/rules.mk
index 49044c8cfb..7d3516bc42 100644
--- a/rules.mk
+++ b/rules.mk
@@ -254,4 +254,16 @@ all:
 FORCE: ;
 .PHONY: FORCE
 
+val.%:
+	@$(if $(filter undefined,$(origin $*)),\
+		echo "$* undefined" >&2, \
+		echo '$(subst ','"'"',$($*))' \
+	)
+
+var.%:
+	@$(if $(filter undefined,$(origin $*)),\
+		echo "$* undefined" >&2, \
+		echo "$*='"'$(subst ','"'\"'\"'"',$($*))'"'" \
+	)
+
 endif #__rules_inc