Adjust PINGPONGS for timer resolution:
 - Set PINGPONGS to 1 for a high resolution time.
 - Set PINGPONGS to larger number for low resolution timer, e.g. 10.

Adjust REPS for "adequate" statistics.  For many computers,
50 is sufficient.

Adjust LOG2N_MAX for maximum message length.   If the time for the
shortest message length is t_small, and the approximate message
rate is R, then choose LOG2N_MAX as follows:

	t=N_MAX/R
	10=(N_MAX/R)/t_small

	LOG2N_MAX ~= log2(100*t_small*R)

	Example:   t_small ~= 10 microsecond
		   R       ~= 50 Mbyte/s

	LOG2N_MAX ~= log2(100*10*50)
                  ~= log2(50000)
                  ~= 16

If resultant printed times are negative, then this is indication of
timer resolution problems.   In this case, increase PINGPONGS.
