Add support for Visual Studio BuildInParallel custom build tools

https://blogs.msdn.microsoft.com/vcblog/2018/09/18/parallel-custom-build-tools-in-visual-studio-2017/

Visual Studio 2017 15.8 adds opt-in support for the parallel building of multiple custom tools in the same project (via a BuildInParallel tag), including some basic dependency stages.

Adding support for this to cmake would help vs build times on some projects (e.g. Clang/LLVM) a great deal.