Get rid of dependency on Visual Studio Dev Cmd Window

[yaronyg]

When we do a full gateway rebuild the build scripts depend on some environment variables that are defined in the Visual Studio Dev command window. This has bitten me a couple of times where it turns out I'm not in one of those windows. So I'd like to get rid of this dependency.

This is really an all up question on how to run VS Code without VS. See https://blogs.msdn.microsoft.com/vcblog/2016/10/24/building-your-c-application-with-visual-studio-code/ for example.