Check for define-obsolete-function-alias before using it.

This is for Emacs 21 compatibility.

lisp/org.el

@@ -4799,11 +4799,12 @@ even level numbers will become the next higher odd number."
 	    ((< change 0) (max 1 (1+ (* 2 (/ (+ level (* 2 change)) 2))))))
     (max 1 (+ level change))))
 
-(if (or (featurep 'xemacs) (< emacs-major-version 23))
-    (define-obsolete-function-alias 'org-get-legal-level
-      'org-get-valid-level)
-  (define-obsolete-function-alias 'org-get-legal-level
-    'org-get-valid-level "23.1"))
+(if (boundp 'define-obsolete-function-alias)
+    (if (or (featurep 'xemacs) (< emacs-major-version 23))
+	(define-obsolete-function-alias 'org-get-legal-level
+	  'org-get-valid-level)
+      (define-obsolete-function-alias 'org-get-legal-level
+	'org-get-valid-level "23.1")))
 
 (defun org-promote ()
   "Promote the current heading higher up the tree.