Quieten without-x, without-xml2 builds

* lisp/help.el (x-display-pixel-height, x-display-pixel-width):
* lisp/image.el (image-flush, image-size):
* lisp/textmodes/sgml-mode.el (libxml-parse-html-region):
Declare for compiler.

lisp/help.el

@@ -1059,6 +1059,9 @@ is currently activated with completion."
 	  (setq minor-modes (cdr minor-modes)))))
     result))
 
+(declare-function x-display-pixel-height "xfns.c" (&optional terminal))
+(declare-function x-display-pixel-width "xfns.c" (&optional terminal))
+
 ;;; Automatic resizing of temporary buffers.
 (defcustom temp-buffer-max-height
   (lambda (_buffer)

lisp/image.el

@@ -29,6 +29,7 @@
   "Image support."
   :group 'multimedia)
 
+(declare-function image-flush "image.c" (spec &optional frame))
 (defalias 'image-refresh 'image-flush)
 
 (defconst image-type-header-regexps
@@ -1001,6 +1002,8 @@ default is 20%."
               (setq new (nconc new (list key val))))))
           new)))
 
+(declare-function image-size "image.c" (spec &optional pixels frame))
+
 (defun image--current-scaling (image new-image)
   ;; The image may be scaled due to many reasons (:scale, :max-width,
   ;; etc), so find out what the current scaling is based on the

lisp/textmodes/sgml-mode.el

@@ -2232,6 +2232,9 @@ buffer's tick counter (as produced by `buffer-modified-tick'),
 and the CDR is the list of class names found in the buffer.")
 (make-variable-buffer-local 'html--buffer-ids-cache)
 
+(declare-function libxml-parse-html-region "xml.c"
+                  (start end &optional base-url discard-comments))
+
 (defun html-current-buffer-classes ()
   "Return a list of class names used in the current buffer.
 The result is cached in `html--buffer-classes-cache'."