Ok, since there's no unanimous favourite for new command line switch,
the following would work just as well, and still use -V.

$ make -V FUBAR
${FU} ${BAR}
$ make -V '$FUBAR'
fu bar
$

Note the distinct lack of {} after the $, it is just a clue to make
that we want Var_Subst rather than Var_Value.  Also, since '$' can be
a pain to quote/escape in some cases and since ':' is also an illegal
char in a variable name, I'd treat '$' or ':' as the expand clue.

$ make -V :FUBAR
fu bar
$

--sjg