Prerequisite variable expansion

[Casey Liscum]

Variables involved in a target-specific modification do not expand to
the updated form when used as a prerequisite for that target.  They do
expand properly when used in the rule's command structure.

Here is a simplified example Makefile:
-----
# initial target value set;
TARGETS = foo

# set target-specific value of ${TARGETS}
both : TARGETS += bar

# rules
both : ${TARGETS}
        @echo TARGETS ARE: ${TARGETS}

foo bar :
        @echo MAKING TARGET: $@
-----

and here is it's output:
-----
$ make both
MAKING TARGET: foo
TARGETS ARE: foo bar
-----

$(TARGETS) only expands to 'foo' when making, as noted by the lack of a
line 'MAKING TARGET: bar' in the output.
When $(TARGETS) is echoed in the command for 'both', however, it expands
to the value set by the target-specific modification.

I was looking over the documentation to find out if this is an actual
bug, but could not find anywhere that says _when_ in the makefile
target-specific variables are set.  I assumed it was as they are read,
but this doesn't seem to be the case.

I've tested with make versions 3.77, 3.78, and 3.79.
I run the stable distribution of Debian GNU/Linux (potato).

Thanks!

-- 
Casey W. Liscum            Computer Science, Drexel University
If puns were deli meat, this would be the wurst.