It was unintuitive that the withGTK option controlled whether a
fontforge GUI was enabled at all, since fontforge supports multiple
GUIs (a GTK one and an X11 one -- confusingly the X11 one still
requires GTK).
With the new withGUI option, it is possible to build the X11 GUI by
setting withGTK to false and withGUI to true. I haven't added an
attribute for this configuration because I expect it to be pretty
obsure -- my rationale for adding this option was code clarity rather
than utility.
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