[SV 14927] Allow parallel builds for archives

[SV 14927] Allow parallel builds for archives

Compare the timestamp of the object file (if it exists) with the
archived object and if the object file is newer, ensure it's updated
in the archive.

* NEWS: Announce the new capability.
* doc/make.texi (Dangers When Using Archives): Explain how to enable
parallel builds with archives.
* src/remake.c (f_mtime): For archive element files check the mod
time of the object file (if it exists) against the archive object
(if it exists).
* tests/scripts/features/archives: Add tests for this capability.