Assume something more recent than X11R6

* lisp/bindings.el:
* lisp/menu-bar.el:
* lisp/printing.el:
* lisp/thumbs.el (thumbs-conversion-program): Assume we have something
more recent than X11R6.

lisp/bindings.el

@@ -1211,7 +1211,7 @@ if `inhibit-field-text-motion' is non-nil."
 ;; (define-key global-map [kp-9]		'function-key-error)
 ;; (define-key global-map [kp-equal]	'function-key-error)
 
-;; X11R6 distinguishes these keys from the non-kp keys.
+;; X11 distinguishes these keys from the non-kp keys.
 ;; Make them behave like the non-kp keys unless otherwise bound.
 ;; FIXME: rather than list such mappings for every modifier-combination,
 ;;   we should come up with a way to do it generically, something like

lisp/menu-bar.el

@@ -604,7 +604,7 @@ Do the same for the keys of the same name."
   (define-key global-map [f20] 'clipboard-kill-region)
   (define-key global-map [f16] 'clipboard-kill-ring-save)
   (define-key global-map [f18] 'clipboard-yank)
-  ;; X11R6 versions:
+  ;; X11 versions:
   (define-key global-map [cut] 'clipboard-kill-region)
   (define-key global-map [copy] 'clipboard-kill-ring-save)
   (define-key global-map [paste] 'clipboard-yank))

lisp/printing.el

@@ -103,14 +103,14 @@ Please send all bug fixes and enhancements to
 ;; For example, after previewing a PostScript file, *Printing Command Output*
 ;; will have the following entry:
 ;;
-;;    /usr/X11R6/bin/gv ("/home/user/example/file.ps")
+;;    /usr/bin/gv ("/home/user/example/file.ps")
 ;;    Exit status: 0
 ;;
 ;; In the example above, the previewing was successful.  If during previewing,
 ;; you quit gv execution (by typing C-g during Emacs session), the log entry
 ;; would be:
 ;;
-;;    /usr/X11R6/bin/gv ("/home/user/example/file.ps")
+;;    /usr/bin/gv ("/home/user/example/file.ps")
 ;;    Exit status: Quit
 ;;
 ;; So, if something goes wrong, a good place to take a look is the buffer

lisp/thumbs.el

@@ -95,10 +95,11 @@ When it reaches that size (in bytes), a warning is sent."
   (if (eq system-type 'windows-nt)
       "convert.exe"
     (or (executable-find "convert")
-	"/usr/X11R6/bin/convert"))
+        "/usr/bin/convert"))
   "Name of conversion program for thumbnails generation.
 It must be \"convert\"."
-  :type 'string)
+  :type 'string
+  :version "28.1")
 
 (defcustom thumbs-setroot-command
   "xloadimage -onroot -fullscreen *"