Use OpenMP to parallelize the iterations in "main.cc".

On my quadcore laptop, the version of run_HolmesBot built with
"FAST=1 OPENMP=1" completes in 193 seconds, as opposed to 466 seconds
for "FAST=1 OPENMP=0".

run_CheatBot must be built with "FAST=1 OPENMP=0", unless you
are using a compiler which supports OpenMP's "#pragma threadprivate"
directive for non-POD types. No such compiler seems to exist for
Mac OS X at the moment.