In my opinion, the problem in creating a cross-platform project is mainly in the choice of libraries. I would suggest that OpenGL and SDL would be sufficient. I would also like to consider enabling Boost for those parts that are not provided by SDL.
As for CMake: it is a good build tool for both Windows and Mac. If you are new to CMake, check out this question , but also look at a well-known engine like Ogre and how they set up their project.
I would advise you to create good CMakeLists files, and then it really doesn't matter which IDE you use. You can use Xcode or simple Make files on your Mac using Visual Studio or Make files (or Eclipse) on Windows. Perhaps you can even set up a small Linux PC (for example, with Virtual Box and Ubuntu), and you can also test your cross-platform on Linux.
As for Git: the biggest advantage is that you have your own local repository where you can fork and merge everything you need without having to access some central repository. Make sure you keep track of some central branch.
Andrรฉ
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