Two 'make -p' timestamp issues

[Paul Eggert]

In some unusual, timing-dependent cases on some platforms, 'make -p' can 
generate output that incorrectly makes it look like the system clock ran 
backwards, when it didn't.

Also, there's a torture-test case (system clock in the far past or 
future) where 'make -p' can dump core.

Proposed patch attached.

0001-Fix-clock-skew-crash-issue-with-make-p.patch
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