Use CMake's `ExternalProject` Feature
CMake has a feature to build external projects from within a project, see the documentation for more information.
Currently, the app ships with some 3rd party dependencies, which we usually build by just checking them out and building them as part of the app build itself. However, this has some not-so-nice side effects (in particular, every option and variable of such a dependency becomes visible in the app interface as well).
Hence, it might be a better solution to use the external project feature. In particular, it would allow us to more easily integrate part of the build which currently are driven by additional build scripts (like building OpenSSL for Android).