Use select-message-coding-system in mh-comp

* lisp/mh-e/mh-comp.el (mh-send-letter):
Use select-message-coding-system.  (Bug#30060)

lisp/mh-e/mh-comp.el

@@ -305,17 +305,19 @@ message and scan line."
         (file-name buffer-file-name)
         (config mh-previous-window-config)
         (coding-system-for-write
-         (if (and (local-variable-p 'buffer-file-coding-system
-                                    (current-buffer)) ;XEmacs needs two args
-                  ;; We're not sure why, but buffer-file-coding-system
-                  ;; tends to get set to undecided-unix.
-                  (not (memq buffer-file-coding-system
-                             '(undecided undecided-unix undecided-dos))))
-             buffer-file-coding-system
-           (or (and (boundp 'sendmail-coding-system) sendmail-coding-system)
-               (and (default-boundp 'buffer-file-coding-system)
-                    (default-value 'buffer-file-coding-system))
-               'iso-latin-1))))
+         (if (fboundp 'select-message-coding-system)
+             (select-message-coding-system) ; Emacs has this since at least 21.1
+           (if (and (local-variable-p 'buffer-file-coding-system
+                                      (current-buffer)) ;XEmacs needs two args
+                    ;; We're not sure why, but buffer-file-coding-system
+                    ;; tends to get set to undecided-unix.
+                    (not (memq buffer-file-coding-system
+                               '(undecided undecided-unix undecided-dos))))
+               buffer-file-coding-system
+             (or (and (boundp 'sendmail-coding-system) sendmail-coding-system)
+                 (and (default-boundp 'buffer-file-coding-system)
+                      (default-value 'buffer-file-coding-system))
+                 'iso-latin-1)))))
     ;; Older versions of spost do not support -msgid and -mime.
     (unless mh-send-uses-spost-flag
       ;; Adding a Message-ID field looks good, makes it easier to search for