[m-dev.] for review: use available opt files

[Simon Taylor]

> > Add options --use-opt-files and --use-trans-opt-files, which cause
> > the compiler to use any `.opt' or `.trans_opt' files which are available
> > when the dependencies are made, but not make any more `.opt' files.
> > This is useful for using just the `.opt' files for the library
> > without building them for the compiler.
> 
> Hmm, for that to work, I think it means you would need to do
> an `mmake' in the library directory before doing an `mmake depend'
> in the compiler directory -- is that correct?
>
> If so, then the top-level Mmakefile and the tools/bootcheck script
> might need modification to work properly with --use-opt-files.

It works without that change - the `.d' files are rebuilt when compiling to C,
so they are updated to include the newly available `.opt' files
in the library. Since they are newly created, they should be up-to-date,
so the lack of a dependency in the original `.d' file shouldn't matter.

Simon.