Support multiple cpus in vsbuild

[KevinSeghetti]

Support for multiple cpus in vsbuild. It is frustrating to have a quad core system and see the cpu usage at 25% on a long build because vsbuild is only using one of them.

(have it build 4 targets at once, like the -j option in GNu Make).

[KevinSeghetti]

vsbuild needs to support parallel builds.

I have a 4 cpu machine. vsbuild only uses one of them. It is frustrating to wait for a build to finish that is only using 25% of the available CPU. This is just going to get worse as we get more CPUs per machine.

I want the equivalent of 'make -j 4'

Thanks for a great product.