To be clear, on Windows _popen works properly in a console application like my demo project. As you note, graphical applications can create a child process. You can always have an application fall-back to zlib if the executable is not found (as my example demonstrates).
Since @ben.boeckel suggested that we restrict ourselves to libs compatible compression, I think pigz is the only game in town for parallel compression. I think my demo demonstrates it is pretty easy to integrate across platforms. This is your project, and you need to determine if the benefit to the user outweighs the development and maintenance time required, as well as your own aesthetic for distributing software. For my own projects I have decided to leverage pigz, but I respect your wishes for your own project.