Warning Levels for Visual Studio's C++ compiler

Copying an issue raised on [exercism/exercism](https://github.com/exercism/exercism/issues/4976) for better visibility:

> In the C++ track, exercises come with a CMakeLists.txt file that configures CMake for each exercise, and I noticed that when building C++ solutions to exercises certain warnings I'd expect to see aren't appearing.
>
> It looks like warning levels are set to be verbose for Clang and GNU compilers but there's no setting that addresses the default compiler MSVS ships with.
>
> https://stackoverflow.com/questions/2368811/how-toset-warning-level-in-cmake/50882216#50882216
>
> This SO answer describes how that could be addressed for visual studio, and it would be a great help to new programmers such as myself if this configuration option could be added.