Don't require Common Lisp extensions at run time

Following Lisp Coding Conventins (Coding Conventions(elisp)),
to avoid name conlicts, we should change

    (require 'cl)

to

    (eval-when-compile (require 'cl))

Thanks to Shigeru Fukaya for pointing this out.

Author: jrblevin

markdown-mode.el

@@ -366,6 +366,8 @@
 ;;     escape shell commands.
 ;;   * Christopher J. Madsen <[email protected]> for patches to fix a match
 ;;     data bug and to prefer `visual-line-mode' in `gfm-mode'.
+;;   * Shigeru Fukaya <[email protected]> for better adherence to
+;;     Emacs Lisp coding conventions.
 
 ;;; Bugs:
 
@@ -403,7 +405,7 @@
 
 (require 'easymenu)
 (require 'outline)
-(require 'cl)
+(eval-when-compile (require 'cl))
 
 ;;; Constants =================================================================