No way to consume ios/osx versions from CMake

I noticed it appears that the `CMakeLists.txt` isn't maintained when making changes to the `xcodeproj`. Shouldn't the `CMakeLists.txt` be modified to generate it and if so, would the maintainers be open to a PR making this change?

I think the workaround otherwise would be to `execute_process(xcodebuild...)` and link to the resulting library from CMake.