CMake example for SGDK via marsdev

A quick hacky setup for using CMake for developing SGDK based projects.

I use SGDK via the marsdev system as it provideds a consistant experience for me, please ensure you have marsdev installed.

You will need to either add a MARSDEV define in the call to CMake to the location where marsdev is installed, or add a MARSDEV environment variable to your shell.

Note!

I only use macOS, so this is tested only under macOS, it may not work right on other platforms