gtk_entry_get_buffer() is available since GTK 2.18 only. When you try to add that declaration inside your own module, of course you have to ensure that const lib is set to valid library name. That may be not easy, because const lib may be not exported from GTK2 module. Maybe try to patch that module?
I guess GTK3 module will have gtk_entry_get_buffer -- if not let me know, I will fix it then.
