Well, = doesn't really behave like traditional variable binding, since you can do stuff like:

    build = cc -c -o $@ $<
    file1.o: file1.c
        $(build)
    file2.o: file2.c
        $(build)

(This is meant to illustrate the difference, this isn't a good makefile.)

To reinforce the notion that variables are really functions, you can even supply arguments when you call them:

    # reverse is a function
    reverse = $(2) $(1)

    # eventually expands to 2 1
    a = $(call reverse,1,2)