Make this compile with my gcc 4.8.2

Signed-off-by: Uli Schlachter <psychon@znc.in>
Add some CMake + GTK+3 sample code

This was only tested on linux (debian testing), but I guess that using GTK+3 is
waaaaaaaaay easier than directly using Xlib or xcb. Really, a lot easier.

Signed-off-by: Uli Schlachter <psychon@znc.in>