Fixing Xcode 5 warning

Xcode 5 has gotten more pedantic about undeclared selectors, by
default.

src/cocoa_window.m

@@ -783,7 +783,8 @@ static void createMenuBar(void)
 
     // Prior to Snow Leopard, we need to use this oddly-named semi-private API
     // to get the application menu working properly.
-    [NSApp performSelector:@selector(setAppleMenu:) withObject:appMenu];
+    SEL setAppleMenuSelector = NSSelectorFromString(@"setAppleMenu:");
+    [NSApp performSelector:setAppleMenuSelector withObject:appMenu];
 }
 
 #endif /* _GLFW_USE_MENUBAR */