another Makefile fix

[Henry Cejtin]

Sorry: I meant the old Makefile.
As to the Makefile hack, if the only point is to be able to make mlton.cm, then
I would vote (not strongly) to just have a separate file (make-cm or something
like that) which makes mlton.cm.  This seems better to me then the DWIM'ishness
in the Makefile where it isn't clear what cases it can handle and which ones
it can't.
I.e., no Makefiles make any .cm files and there is a separate executable which
you run when you know that hte .cm file has to be remade (and, I suppose, it
depends on either cmcat or mlton -f).