Should be USE="v4l" instead. And looks like the whole USE flag is ignored, nothing in CMakeLists.txt: ssuominen@null /tmp/phonon-backend-vlc-0.4.1 $ grep -r -i v4l * vlc/mediaobject.h: * \li capture devices (V4L) vlc/mediaobject.cpp: if (driverName == QByteArray("v4l2")) { vlc/mediaobject.cpp: loadMedia(QLatin1Literal("v4l2://") % deviceName);
It just supports the case when v4l2:// protocol works (I think some kdelibs or kdemultimedia component should provide that). Nothing v4l specific in phonon-vlc besides this so this USE flag and dependencies shlould be probably removed.
v4l support was removed: https://projects.kde.org/projects/kdesupport/phonon/phonon-vlc/repository/revisions/59d4ee18ea8ac5380ec7688f2f104c4b6ccf9a9e