"no rule to make target" when target is a variable

[Brendan Bridgford]

I'm trying to use a variable as a target without much luck.  My makefile contains:

.PHONY = foobar
foobar : a = foo
foobar : b = bar
c=$(a)$(b)

$(c) :
    echo $(c)

When I invoke

>make foobar

... it returns:

make: *** No rule to make target `foobar'.  Stop.

However it works if I replace the variable definition "c=$(a)$(b)" with   "c=foobar":

.PHONY = foobar
foobar : a = foo
foobar : b = bar
c=foobar

$(c) :
    echo $(c)

Now I invoke

>make foobar

... it returns:

echo foobar
foobar

Why isn't it working when I use the variable $(c) as a target?  I'm using GNU Make 3.80 for i686-pc-cygwin.

thx,
--Brendan