Add wxUSE_MARKUP and wrap SetLabelMarkup() in it.

Make it possible to disable all the new markup-related code by setting
wxUSE_MARKUP to 0 in setup.h or using configure --disable-markup option.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67062 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/stattext.h

@@ -56,7 +56,9 @@ protected:
 
     virtual wxString DoGetLabel() const;
     virtual void DoSetLabel(const wxString& str);
+#if wxUSE_MARKUP
     virtual bool DoSetLabelMarkup(const wxString& markup);
+#endif // wxUSE_MARKUP
 
 private:
     // Common part of SetLabel() and DoSetLabelMarkup().