I noticed that when using linux, if you somehow (sdl, xgl, ...) include X.h AND cegui/elements/CEGUIListHeaderSegment.h, you will have a naming collision:
None
is a #define in X.h and a enum in CEGUI.
I solved this problem for my by playing around with my includes, but this could cause more trouble to others. I wonder that nobody complained about it yet.
Don't know if you can change this ... this would be a breaking change
I saw that Qt defines a "None" somewhere too ...