How to add parallel build support for the rule of boost_build

[fuhailin]

I have tried to use the rule of boost_build, and it's convenient to build the boost library, but it consumes my 2323.782 seconds, about 38 minutes to build the while boost library on my 12 cores Macbook.

During the building process, I noticed that not all of my computer cores are working, so I guess if we can add parallel build support for the rule of boost_build, less time will consume when building boost.

So is there a way to pass a '-jnproc' argument to the boost_build, just like cmake and configure_make rules according to #329 .

If the answer is yes, could you please provide a demo?