another Makefile fix

[Stephen Weeks]

> I suggest that no make entry depend on any .cm file, and that the entry for
> the .cm file be
> 	$(NAME).cm:
> 		(						\
> 			echo 'Group is' &&			\
> 			...					\
> 		) >$(NAME).cm
> This way there is no Make DWIMishness, and the only way to have the cmcat
> stuff run is to explicitly run `make mlyacc.cm'.

This fails in two ways.

1. $(NAME) does depend on $(NAME).cm.  If you take this dependence out, and do 
a make where $(NAME).cm does not exist, then the call to $(MLTON) $(NAME).cm
will fail.

2. $(NAME).cm does depend on src/yacc.lex.sml.  If you take out this dependence,
and do a clean make, then the call to cmcat will fail.

I'm happy with what Matthew has checked in right now.