boost jam problem with parallel builds

[alx_s]

OK, this bug makes my life miserable for a long, long
time. :)

In short, if I have an action which updates more then
one target then I have problem with parallel build.

Attached is a simple testcase. Just run 'jam -j2' and
you can notice that target 'g2.generated' is been used
before it is updated (created).

The testcase has just 2 rules - 'Gen' models generating
2 files which are used by 'Use'.
The testcase doesn't actually create any files, but
that doesn't matter. Jam works with targets, not files.

The last line in the Jamfile is a workaround. If you
uncomment it, the test works fine.

I spent some time trying to find the reason and to fix
the bug, but succeed only in the first part.

I can discuss it in more details with someone
interested in fixing it. I just didn't get enough time
to fix the bug by myself.