Provided Basic Support for -l N option on windows platform.

I have implemented basic support for **-l N** on windows platform.

Tested with platform==msvc and platform==mingw.

Values returned from OS are filtered with basic filter, to minimize load variation caused by short time between consecutive calls.

I have tested it on 2 core machine with HT and 4 core one, and CPU load in Task Manager matches one provided in -l option.

Any suggestion and improvements are moe than welcome.


`GetSystemTimes` is XP SP1 and newer. Do you know if ninja currently runs on older XPs?