how to decide design and build process for a large cross platform c++ project? [closed]

I'm going to be starting a large scale software project involving a cross-platform c++ dynamic library.

1. What would be a good way to structure your files for a cross platform dynamic library, which is useful and efficient for building purpose?
2. What should be the choice of build process? Should it be CMAKE or should it be platform dependent IDE's like visual studio in windows or Xcode in MAC to make it more easy or efficient?
3. How to go through the process of finding the best design for a large scale c++ project?
4. How important is to start with multithreading and mutex locks from the beginning of the project itself, as they are needed for the project as it is a cross platform dynamic library!, But my question is how much necessary or feasible to start adding these in the beginning of the project and make it complicated? what is the general perspective about that?

Anything to be taken care of when developing/building a cross platform c++ dynamic library? Any kind of help is appreciated!!