Add #if checks fixing minimal wxGTK build.

Check for functions availability before using them. This fixes compilation of
wxGTK with all features disabled.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66091 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/wincmn.cpp

@@ -2739,8 +2739,10 @@ void wxWindowBase::OnMiddleClick( wxMouseEvent& event )
         else
 #endif // __WXDEBUG__
         {
+#if wxUSE_MSGDLG
             // just Ctrl-Alt-middle click shows information about wx version
             ::wxInfoMessageBox((wxWindow*)this);
+#endif // wxUSE_MSGDLG
         }
     }
     else