This is PR/1696 in the GNU make bug database. A patch is included in
the resolution (essentially the same fix you used).
You can find info on accessing the GNU make bug database in the README
in the GNU make distribution.
Thanks for the report!
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There is a bug in make-3.79 which was not present in 3.77. I have
confirmed the problem under IRIX 6.5 and SunOS 5.5.1, but I believe it
affects all OSes supporting archives.
When using implicit rules for library archiving, the timestamp is computed
incorrectly for the (not yet existing) target
