请问为什么“make clean”要先加上前缀然后再替换掉呢？

# Directories & files removed with 'make clean'

CLEAN_DIRS  +=

CLEAN_FILES +=	myapp



# clean - Delete most

#

clean: rm-dirs  := $(CLEAN_DIRS)

clean: rm-files := $(CLEAN_FILES)

clean-dirs      := $(addprefix _clean_, . $(myapp-dirs))



PHONY += $(clean-dirs) clean archclean

$(clean-dirs):

	$(Q)$(MAKE) $(clean)=$(patsubst _clean_%,%,$@)



clean: $(clean-dirs)

	$(call cmd,rmdirs)

	$(call cmd,rmfiles)

	@find . $(RCS_FIND_IGNORE) \

		\( -name '*.[oas]' -o -name '.*.cmd' \

		-o -name '.*.d' -o -name '.*.tmp' \

		-o -name '.tmp_*.o.*' \

		-o -name '*.gcno' \) -type f -print | xargs rm -f