
Fix public issue #38: Make the microbenchmark framework handle
properly cases where gettimeofday() can stand return the same
result twice (as sometimes on GNU/Hurd) or go backwards
(as when the user adjusts the clock). We avoid a division-by-zero,
and put a lower bound on the number of iterations -- the same
amount as we use to calibrate.

We should probably use CLOCK_MONOTONIC for platforms that support
it, to be robust against clock adjustments; we already use Windows'
monotonic timers. However, that's for a later changelist.

R=csilvers
DELTA=7  (5 added, 0 deleted, 2 changed)


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gburd/snappy,gburd/snappy,gburd/snappy,gburd/snappy