gtk_dialog_get_widget_for_response requires gtk+ 2.20; not checked at configure

The function 'gtk_dialog_get_widget_for_response' is used in the new GTK+2.0 native file dialog theme unconditionally, but it appears this function was introduced in GTK+ 2.20 – systems using older versions (e.g. RHEL / CentOS 6 which ship with version 2.18) will pass the configure check, but will fail to compile on qgtk2dialoghelpers.cpp due to the missing function declaration. It seems that either a GTK version check should be added, or the function should be modified to work against older versions of GTK+2.0.

As a side note, bypassing these two calls allows the rest of Qt 5.1 alpha to compile and link successfully.