since we're using ${X11_LIBRARIES} just about everywhere.

This is currently being found indirectly by one or two paths: via
Qt4ConfigDependentSettings.cmake or FindOpenGL.cmake. The former is a
ticking time-bomb, since the one in upstream CMake does *not* do
find_package(X11). The latter is also a bomb since OpenGL is optional
in kde-workspace (it works with OpenGL/ES).