Fix setf on emacs23 (#205)

`gv-define-simple-setter` is not defined on Emacs 23, so we need to use
the macro `defsetf` on Emacs 23. `defsetf` is not autoloaded, so we must
require 'cl before using it. We don't need to load 'cl on later Emacs
versions.

However, the whole form is macro-expanded on all Emacs versions. Emacs
24+ assumes that `(defsetf ...)` is a function call because it hasn't
loaded 'cl. This generates byte-compile warnings about unused variables
and nonexistent functions, so silence those.

dash.el

@@ -578,14 +578,26 @@ Alias: `-any'"
 
 \(fn LIST)")
 
-(gv-define-simple-setter -first-item setcar)
+;; TODO: emacs23 support, when dropped remove the condition
+(eval-when-compile
+  (require 'cl)
+  (if (fboundp 'gv-define-simple-setter)
+      (gv-define-simple-setter -first-item setcar)
+    (require 'cl)
+    (with-no-warnings
+      (defsetf -first-item (x) (val) `(setcar ,x ,val)))))
 
 (defun -last-item (list)
   "Return the last item of LIST, or nil on an empty list."
   (declare (pure t) (side-effect-free t))
   (car (last list)))
 
-(gv-define-setter -last-item (val x) `(setcar (last ,x) ,val))
+;; TODO: emacs23 support, when dropped remove the condition
+(eval-when-compile
+  (if (fboundp 'gv-define-setter)
+      (gv-define-setter -last-item (val x) `(setcar (last ,x) ,val))
+    (with-no-warnings
+      (defsetf -last-item (x) (val) `(setcar (last ,x) ,val)))))
 
 (defun -butlast (list)
   "Return a list of all items in list except for the last."