Correct the OS X branch of the clipboards ifdef.

That's what I get for not testing on all platforms before I push.
Forgot that, since OS X GTK mimics X11 GTK closely enough to still use
the name "CLIPBOARD" for the unique system clipboard, I had left this
code base's internal name for it as CLIP_CLIPBOARD and not the
CLIP_SYSTEM I used on Windows.

unix/unix.h

@@ -141,8 +141,8 @@ unsigned long getticks(void);
 #define CLIPUI_DEFAULT_AUTOCOPY FALSE
 #define CLIPUI_DEFAULT_MOUSE CLIPUI_IMPLICIT
 #define CLIPUI_DEFAULT_INS CLIPUI_EXPLICIT
-#define MENU_CLIPBOARD CLIP_SYSTEM
-#define COPYALL_CLIPBOARDS CLIP_SYSTEM
+#define MENU_CLIPBOARD CLIP_CLIPBOARD
+#define COPYALL_CLIPBOARDS CLIP_CLIPBOARD
 #else
 #define MOUSE_SELECT_CLIPBOARD CLIP_PRIMARY
 #define MOUSE_PASTE_CLIPBOARD CLIP_PRIMARY