Feat: setup vc environment during configuration

This would allow us to build using ninja from the command line.
This is currently not possible as CMake cannot find `cl` when building from a "normal" shell.
The vcvarsall script would have to be called before.
This is very inconvenient.
With this change the vc environment would be setup during configuration.
However, as discussed [here](https://github.com/MarkSchofield/WindowsToolchain/issues/58) this does not work correctly yet.