Now add some dependencies, some required, some optional, for all platforms, or some specific to some platforms.

Then add configuration options for conditional compilation, and a build system to define them properly.

Then use the configuration options of your dependencies in your build system.

Finally, repeat this process for every project and make sure you use the same syntax/api for your build system so you don't have to relearn everything everytime.

cc main.c is as easy as it gets when you rewrite everything from scratch.

cargo is as easy as it gets when you want an ecosystem.