bug#29713: 26.0; Provide completion for `make-frame-on-display'

[Juri Linkov]

>> Maybe it could use `x-display-list'.  But I don't know how portable it is.
>> Is `x-display-list' available on OSX and Windows?
>
> It's not available on --without-x builds:
>
> 0. ./src/emacs -Q
> 1. M-x toggle-debug-on-error RET
> 2. M-x make-frame-on-display RET
>    Debugger entered--Lisp error: (void-function x-display-list)
>
> What's TRT here?

Maybe fall back to previous implementation replaced by x-display-list
in 0746a9e275d, i.e.

  (if (fboundp 'x-display-list)
      (x-display-list)
    (delete-dups
     (mapcar (lambda (frame)
               (frame-parameter frame 'display))
             (frame-list))))

Does this fail on --without-x builds?