[m-rev.] for review: fix mmake rules for os,cs,css,javas

[Zoltan Somogyi]

On Sat, 11 Apr 2020 19:13:26 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> To build the source distribution we use the script tools/build_srcdist.
> It will invoke "mmake tar" as well as do all the other necessary stuff.
> (Note that it will do a "git clean" on whatever workspace it is invoked
> in.)

That was a good idea; it revealed a misplaced backslash.
But after fixing that, the building of the source distribution
succeeded, so I am committing the change.

Zoltan.