[SV 44742] Fix double-colon rules plus parallel builds.

* remake.c (update_file): Don't update double-colon target status
if we're still building targets.
(ftime_t): Don't propagate timestamps for double-colon targets that
we've not examined yet.
* tests/scripts/features/double_colon: Add parallel build tests.

Copyright-paperwork-exempt: yes
2 files changed