Add wxUSE_MENUBAR build option, off by default in wxiOS

Allow building without wxMenuBar (but with wxMenu), as this class
doesn't exist and can't be reasonably implemented under iOS (but
wxMenu can and should be, as it's widely used in iOS 14 UI).

include/wx/menu.h

@@ -282,6 +282,7 @@ public:
     // handler to use for these events, but this should never be needed.
     void UpdateUI(wxEvtHandler* source = NULL);
 
+#if wxUSE_MENUBAR
     // get the menu bar this menu is attached to (may be NULL, always NULL for
     // popup menus).  Traverse up the menu hierarchy to find it.
     wxMenuBar *GetMenuBar() const;
@@ -292,6 +293,7 @@ public:
 
     // is the menu attached to a menu bar (or is it a popup one)?
     bool IsAttached() const { return GetMenuBar() != NULL; }
+#endif
 
     // set/get the parent of this menu
     void SetParent(wxMenu *parent) { m_menuParent = parent; }
@@ -443,6 +445,8 @@ WX_DECLARE_EXPORTED_LIST(wxMenuInfoHelper, wxMenuInfoHelperList );
 // wxMenuBar
 // ----------------------------------------------------------------------------
 
+#if wxUSE_MENUBAR
+
 class WXDLLIMPEXP_CORE wxMenuBarBase : public wxWindow
 {
 public:
@@ -584,6 +588,7 @@ protected:
 
     wxDECLARE_NO_COPY_CLASS(wxMenuBarBase);
 };
+#endif
 
 // ----------------------------------------------------------------------------
 // include the real class declaration