Limiting expansion of make variables

[Richard Cavell]

   Hello all.  For debugging purposes, I am trying to put this into my
   makefile:

   .PHONY: dumpvars

   dumpvars: source.c source.h
   @echo "$(RM) is " $(RM)
   @echo "$? expands to : " $?

   and so on.  But obviously, Make is expanding all instances of the
   variables' names.  How do I prevent this?

   TIA,
   Richard