How to create "re-entrant" makefiles?

[David Wuertele]

I want to include a makefile multiple times, and I each time I want it to define
rules based on the current value of some variables.  But I'm stymied by the fact
that Make only interpolates variable values for command scripts when they are
executed, not when they are defined.

For example:

Makefile
--------

MY_VAR := one
include variabletest.mk

MY_VAR := two
include variabletest.mk

variabletest.mk
---------------

$(MY_VAR):
        echo $(MY_VAR) > $(MY_VAR)

all: $(MY_VAR)


When I type "make", I expect it to do this:

  echo one > one
  echo two > two

But the rules for "one" and "two" both contain $(MY_VAR), which isn't evaluated
until execution time, and at that point MY_VAR contains "two".  So all I see is
this:

  echo two > two

How can I make make do what I want?