when MAX_CONCURRENT_THREAD_NUMBER is not specified, tool will automatically detect number of processor threads. Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=775 Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com> Reviewed-by: Liming Gao <liming.gao@intel.com>