bar = true example

On (info "(make) Conditional Syntax")

          bar = true
          foo = bar
          ifdef $(foo)
          frobozz = yes
          endif

     The variable reference ‘$(foo)’ is expanded, yielding ‘bar’, which
     is considered to be the name of a variable.  The variable ‘bar’ is
     not expanded, but its value is examined to determine if it is
     non-empty.

OK, pound it home by adding: "The above example would work exactly the
same if bar = false or bar = bla."