I think adding external projects to a C++ project is a major decision that should be taken very carefully and the amount of research you need to do to make such

I think adding external projects to a C++ project is a major decision that should be taken very carefully and the amount of research you need to do to make such a decision pales in comparison with just how exactly you integrate it in your project so you can #include its headers and link against its libraries. Then once all that work has been done it's now part of the internal repository so using it with any number of internal projects should be quite easy.