Add BuildRequires: pkgconfig(systemd)
    
    See https://github.com/JoseExposito/touchegg/blob/e7577a22fbce1e42634c8a16c6cb2b39a377e0f6/CMakeLists.txt#L94-L98. The package uses the systemdsystemunitdir variable.
    
    This used to work fine without an explicit dependency. I suspect that
    systemd used to be pulled in by something else in the buildroot or the
    package's dependency chain.