build: override prefix on all system install locations
Following https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/ Unfortunately, the path for systemd user units cannot be overriden, so use a path made from our libdir path. This allows for trouble free builds by just defining prefix. If installing onto system paths was desirable on some setups, it might return through a -Duse_system_paths boolean build option that disables the overrides, but this is a preferrable default.
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