Allow to build and work without pipewire.
But it does not prevent "silent grab" of libpipewire-0.3.so:multimedia/pipewire and libKPipeWire.soaudio/plasma6-kpipewire (USE_KDE=kpipewire).
To prevent "silent grab" it is necessary to patch CMakeList.txt for each port in list:
- x11-wm/plasma6-kwin
- x11/plasma6-plasma-desktop
- x11/plasma6-plasma-workspace
- x11/plasma6-plasma