Re: GTK build doesn't set the `x-toolkit' feature

[Jan D.]

> > x-toolkit means uses Xt, which GTK does not.  Can you give an
> > example of a customization?
>
> A couple of default faces look rather ugly in the non-toolkit build.
> For example the gray and 3D appearance of `tool-bar' and `mode-line'
> look out-of-place in this flat, black & white world.
>
> > Can you use (featurep 'gtk)?
>
> Not in a `defface'.

I think your first point is correct after all, the GTK build should set
x-toolkit.  Using Xt is more of an internal thing in the Emacs sources.
I've checked in a change so that x-toolkit is set for GTK and also added
  (and (memq 'gtk options)
       (featurep 'gtk))

to face-spec-set-match-display so you can have gtk as a type in the 
display
specification.

        Jan D.