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.

Speaking of, why both -ansi and -std=99 [sic[1]]?

[1] should be -std=c99

Because CFLAGS may not be set in the environment, resulting in concatenation to an undefined variable error.

My guess is that the left hand side is a make variable while the right hand side is an environment variable. However the make manual says “Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value.“ so maybe it’s not necessary?...