build: zdtm -- Add implicit rules into zdtm building

Current zdtm building procedure depends on impilcit
make fules, so bring them back until we moved to
nmk build engine.
Reported-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Makefile

@@ -152,11 +152,11 @@ docs:
 PHONY += docs
 
 zdtm: all
-	$(Q) $(MAKE) -C test/zdtm all
+	$(Q) MAKEFLAGS= $(MAKE) -C test/zdtm all
 PHONY += zdtm
 
 test: zdtm
-	$(Q) $(MAKE) -C test
+	$(Q) MAKEFLAGS= $(MAKE) -C test
 PHONY += test
 
 tar-name := $(shell git tag -l v$(CRIU_VERSION))