build: Don't fail immediately when logind is missing

We require logind for the native backend, but the backend itself is optional. However since commit 06c357d7, we will always throw an error if neither logind nor elogind are available, even when the backend is disabled.

As we still support "auto" - that is, whether the native backend is enabled depends on whether its dependencies are available - the easiest option is to make sure we always include either elogind or libsystemd in the dependency check rather than erring out explicitly if neither is found.