[c++] warning: race causing test to hang occasionally

[Gordon Sim]

I noticed our cruisecontrolled build of the cpp trunk was blocked this 
morning. It turned out to be a test that failed to complete (perftest).

I couldn't reproduce it when running make check manually on the same 
box, but I saw the same thing again on the first attempt to restart 
cruisecontrol (a second attempt passed).

Its fairly hard to reproduce reliably, at least for me. Am investigating 
 further, so far without a great deal of success (unfortunately any 
logging or debug seems to mask the problem), but thought it was worth 
mentioning.