another Makefile fix

[Matthew Fluet]

> > Note, this will fail if the person does
> >         make some-target mlyacc.cm some-other-target
> > and the like.
> 
> This will now fail differently.  Unless mlyacc.cm is the only target, then
> it won't be a valid target.  So the above would fail, which I think is
> appropriate.

Actually, it won't fail.  We'll get 
make: Nothing to be done for `mlyacc.cm'.
But, that's the best we can hope for for an existing file that has no
associated rule.

Anyways, unless Henry has really strong objections, I'd still like to use
make to generate them, rather than additional scripts.