I skimmed the comments to your question on stardot.
Seems to me that anyone with the skills to create and maintain a compiler, a compiler that can be built and used on many platforms, will be able to get the hang of git and github in short order.
I do appreciate the difficulty of supporting multiple platforms. Maybe there is a make file for Linux, maybe there is Visual Studio project for Windows, may be there is whatever one needs for Mac, iOS, Android. Etc, etc.
Given your code is written in C then any actual code variations from platform to platform can be accommodated with #defines and #ifdefs and options in the build systems.
It's messy but that is the nature of the beast.
One approach to getting answers as to how to structure and organize all of this is to just put everything you have, source files, make files, project files, manifests, etc into git and publish it on git hub. With suitable build instructions for each platform.
Then, anyone interested can try it out and if they have problems or ideas for a better way to do it they can notify you via an issue report or even propose a "pull request" from their clone of your repository.
Even if people are not collaborating on the developing the actual code they can be helpful in answering the questions you asked on startdot about structuring and build systems.
Memory in C++ is a leaky abstraction .