[Cmake] Moving from gtk1 to gtk2

[Brad King]

> However, this isn't currently clear because wxArt2D's Cmake constructed
> makefiles still use both gtk1 and glib1 include paths erroneously (my
> wxWindows build didn't).
[snip]
> I've attempted defining the GTK_INCLUDE_DIR environment variable as
> "/usr/include/gtk-2.0/include", and various other values, with no success.

When you build wxArt2D, use ccmake to edit the GTK_INCLUDE_DIR and related
variables to point to whatever you want.  GTK_INCLUDE_DIR is a cmake cache
entry, not an environment variable.

-Brad