How to make KDE4 System Settings app work if kde-workspace is in a custom install prefix?

For various reasons I need to use KWin from KDE4 on Ubuntu 18.04. I've compiled kde-workspace-4.11.22 package from source, having installed all the required development packages from the Ubuntu repositories. To avoid messing with the system files, I chose the install prefix to be ~/opt/kde4. Then KWin works fine if I launch it with

PATH=~/opt/kde4/lib/kde4/libexec/:$PATH ~/opt/kde4/bin/kwin

If I simply run it without adding libexec to PATH, I get no compositing because KWin can't check that it works. So far so good. But I also need to tweak some KDE4-specific KWin settings (e.g. window decoration style, buttons), and kde-workspace does have the systemsettings app. I launch it, and get an error: > System Settings was unable to find any views, and hence has nothing to display. So my question is, how to inform System Settings app of the path to the settings plugins? The installation prefix does contain lots of ~/opt/kde4/lib/kde4/kcm_*.so files, but apparently they aren't found.