Fix warnings; close race condition where l->read could be > l->commit.

The race condition can be reproduced when built with gcc (4.2.1) but
doesn't appear to manifest when built with clang (4.0). Caught via
Travis-CI.

Also, tweak no-op benchmark backpressure.
Relax test for clock() delta when queue is idle.

This should be made more strict later.
`make ci`: Run benchmarks verbosely.

Travis-CI is taking a very long time to run the benchmarks on x86-64
Linux when built with clang and I can't reproduce it locally under
same.
fix linker ordering