If turns CFLAGS from a recursive variable to a simple one. Hence it will not be re-expanded every time it is used. This is a speed optimization. $ cat Make

If turns CFLAGS from a recursive variable to a simple one. Hence it will not be re-expanded every time it is used. This is a speed optimization. $ cat Makefile CFLAGS = -Wall $(info $(flavor CFLAGS)) CFLAGS := $(CFLAGS) $(info $(flavor CFLAGS)) $ make recursive simple But TFA says that the author is using := to prevent a recursive definition. It's unclear why the author doesn't just += without doing = or := (unless they want the recursive to simple conversion I talk about above). My guess is the author sligthly misunderstood the handling of the environment inside a Makefile.