Prevent an error with ido-describe-fns and older emacs versions

The metadata for this package specifies that it should only be
installed in Emacs 26 or higher, but people using e.g. el-get can
manually install it on earlier versions. Previously, it would
unconditionally try to use functions that were only defined in Emacs
26, which caused errors in earlier Emacs versions. Now it should just
have no effect on those versions.

ido-describe-fns.el

@@ -17,16 +17,21 @@
 ;;; Commentary: 
 
 ;; This package is an extension for ido-ubiquitous that implements ido
-;; completion for the new `describe-*' family of commands. In recent
-;; Emacs versions, these commands no longer work with ido-ubiquitous
-;; because they now use a function-based collection argument to
-;; implement auto-loading of the file corresponding to the prefix you
-;; entered in order to offer completions of symbols from that file.
+;; completion for the new implementation of the `describe-*' family of
+;; commands. In Emacs 26 and above, these commands no longer work with
+;; ido-ubiquitous because they now use a function-based collection
+;; argument to implement auto-loading of the file corresponding to the
+;; prefix you entered in order to offer completions of symbols from
+;; that file.
 
 ;; Note that there is no separate mode to enable. If
 ;; `ido-ubiquitous-mode' is already enabled, then simply loading this
 ;; package will enable it as well.
 
+;; This package has no effect in Emacs versions earlier than 26, so it
+;; is safe to install it unconditionally in an Emacs config shared by
+;; multiple Emacs versions.
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 
 ;; This program is free software: you can redistribute it and/or modify
@@ -64,10 +69,11 @@ This function auto-loads new files in the same way as
 completion.
 
 Returns non-nil if any new files were loaded."
-  (let ((old-load-history load-history)
-        (prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
-    (help--load-prefixes prefixes)
-    (not (eq load-history old-load-history))))
+  (when (bound-and-true-p help-definition-prefixes)
+    (let* ((old-load-history load-history)
+           (prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
+      (help--load-prefixes prefixes)
+      (not (eq load-history old-load-history)))))
 
 ;; `ido-exhibit' is the ido post-command hook
 (defadvice ido-exhibit (before ido-descfns activate)