#14 Implicit rules for Holmake

[Michael Norrish]

Wouldn't it be nice to be able to write implicit rules?

For example the rule for building C object files would be
something like:

%.o: %.c
<TAB>$(CC) $(CFLAGS) -c $<

While we were at it, we could fix the current built-in
implicit rules for dealing with .sml and Script.sml files.
I think these are wrong. The approach explained in the
GNU make documentation (basically, look for an
instantiation of implicit rules before doing anything),
should be implemented.