@echo "Run 'make' in subdirectories: $(BUILD_SUBDIRS)."
@echo "This top-level make has only 'clean' targets."
-CLEAN_SUBDIRS = $(BUILD_SUBDIRS) examples
+CLEAN_SUBDIRS = $(BUILD_SUBDIRS) examples utils
clean::
for d in $(CLEAN_SUBDIRS); do $(MAKE) -C $$d $@; done