[dewhelmed]

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

[jgrahamc]

No such error occurs.

    $ cat Makefile
    FOO += foo

    all: ; @echo $(FOO)
    $ make
    foo

[dewhelmed]

My bad, I felt like I've seen that error pop up before in my Makefiles, but I should have double-checked before commenting.

[jschwartzi]

There's no such thing as an undefined variable in Make. The variable expands to text. Variables which are not defined by definition expand to no text.

[dewhelmed]

My bad, I felt like I've seen that error pop up before in my Makefiles, but I should have double-checked before commenting.