Don't invoke set-fontset-font if it is void

Prevent Prelude's OS X support from breaking the configuration of
users who do not install a Cocoa version of Emacs.

core/prelude-osx.el

@@ -67,7 +67,9 @@ Windows external keyboard from time to time."
 (menu-bar-mode +1)
 
 ;; Enable emoji, and stop the UI from freezing when trying to display them.
-(set-fontset-font t 'unicode "Apple Color Emoji" nil 'prepend)
+(if (fboundp 'set-fontset-font)
+    (set-fontset-font t 'unicode "Apple Color Emoji" nil 'prepend))
+
 
 (provide 'prelude-osx)
 ;;; prelude-osx.el ends here