What's new in at-spi2-atk 2.5.3:

* Create a library (libatk-bridge), currently with atk_bridge_adaptor_init()
  and atk_bridge_adaptor_cleanup() functions, to facilitate linking directly
  into, ie GTK+ 3.

* Only register events at beginning if an AT is listening (BGO#678475).

* Use XDG_RUNTIME_DIR to hold the socket, rather than a potentially secure
  directory hard-coded under /tmp (BGO#678348).

* Fix various compiler warnings and build errors.