message: Prevent spurious wake up of cond var.
diff: @@ -209,7 +209,7 @@ static void BM_Sequential(int iters) {
  };
  work();
  mutex_lock l(done_lock);
  - if (!done_flag) {
  + while (!done_flag) {
  done.wait(l);
  }
  }
  @@ -232,7 +232,7 @@ static void BM_Parallel(int iters) {
  });
  }
  mutex_lock l(done_lock);
  - if (!done_flag) {
  + while (!done_flag) {
  done.wait(l);
  }
  }
  @@ -259,7 +259,7 @@ static void BM_ParallelFor(int iters, int total, int cost_per_unit) {
  });
  }
  mutex_lock l(done_lock);
  - if (!done_flag) {
  + while (!done_flag) {
  done.wait(l);
  }
  }
  