[cmake] Build library as intended with dune_enable_all_packages

When the enable_all_packages feature was introduced, our CMake minimum requirement did not allow us to add libraries the way it was intended with the feature. The intermediate solution now broke for me when testing with installed libraries (it had temporary build directory paths in its INTERFACE_INCLUDE_DIRECTORIES). Doing it the intended way fixes the problem.

@christi @rhess This should also be backported to the release branch, otherwise PDELab cannot be packaged for Spack.