Improve fix for https://github.com/google/protobuf/issues/295

Requiring the legacy ‘cl’ library unconditionally pollutes the namespace.
Instead, require it only when compiling and in known-broken versions.

This is almost the same patch that opoplawski suggested, except that I removed
the test for ‘emacs-repository-version’, which isn’t defined in Emacs 24.3.

editors/protobuf-mode.el

@@ -64,9 +64,11 @@
 ;;; Code:
 
 (require 'cc-mode)
-(require 'cl)
 
 (eval-when-compile
+  (and (= emacs-major-version 24)
+       (>= emacs-minor-version 4)
+       (require 'cl))
   (require 'cc-langs)
   (require 'cc-fonts))