$(patsubst $(obj)/%,%, $(wildcard \
$(addprefix $(obj)/, *.h */*.h */*/*.h */*/*/*.h))))
-# For GNU Make <= 4.2.1, $(wildcard $(obj)/*/) matches to not only directories
-# but also regular files. Use $(filter %/, ...) just in case.
-clean-dirs += $(patsubst $(obj)/%/,%,$(filter %/, $(wildcard $(obj)/*/)))
+clean-files += $(filter-out Makefile, $(notdir $(wildcard $(obj)/*)))