Keep track of possible targets while we add rules - Later, only
include the command lines / dependencies as needed.

E.g., when defining how to build multi-part objects listed in
$(multi-used-y) and $(multi-used-m), add $(multi-used-y,m) to 
the variable targets. At the end of Rules.make, we then include
the saved command lines for these targets (if they exist), needed
to check whether the command changed and we hence need to rebuild.