This looks like a bug in kde4libs to me, not in plasma-widget-menubar; why doesn't kdelibs5-dev have a dependency on libx11-dev, if its headers include X11/Xlib.h?
This looks like a bug in kde4libs to me, not in plasma- widget- menubar; why doesn't kdelibs5-dev have a dependency on libx11-dev, if its headers include X11/Xlib.h?