[m-rev.] for review: Assign mmake .classes variable conditionally.

[Julien Fischer]

On Tue, 31 Jan 2023, Peter Wang wrote:

> Evaluating the .classes variable in a Mmakefile can be quite slow.
> We can speed up mmake invocations by assigning the variable to an
> empty value unless we are targeting Java.
>
> On my machine, a call to mmake in an up-to-date compiler directory
> now takes about 0.4 seconds, down from 1.2 seconds.
>
> compiler/write_deps_file.m:
>    As above.

That's fine.  Is the .classes variable something that is actually
necessary for mmake --use-mmc-make to work, or is it something that
could just be dropped from Mmakefiles entirely?

Julien.