[CMake] How would I use parallel make on ExternalProjects?

[fatman at crackmonkey.us]

> There is one problem with that, which is what I'm trying to address:
> load balancing.  If your project builds ITK, VTK, and sundry other
> prerequisite libraries, it will spawn 4 sequential builds.  In
> practice this means everything except ITK (which we build with
> wrapping on) finishes, and then ITK chugs along doing its sequential
> build.
> 
> If I do what I'm talking about, I'd sequentially conduct parallel
> builds of ITK VTK etc.  In which case the big hairy libraries, like
> ITK with wrapping, get built in parallel.

Have you considered distcc? It's essentially a compiler load balancer
and fairly mature by now. I don't think it works on Windows though, so
if you need that it might not be what you want.