configure.ac: X_DISPLAY_MISSING is set to 1 if AC_PATH_XTRA fails to find X development files. I don't understand the...

Original commit message from CVS:
* configure.ac: X_DISPLAY_MISSING is set to 1 if AC_PATH_XTRA fails to
find X development files. I don't understand the previous tests and
they fail on my debian/ppc unstable. This one works.
* examples/gstplay/player.c: (main): Set the pipeline to READY before
exiting.
* gst-libs/gst/play/gstplay.c: (gst_play_get_length_callback),
(gst_play_set_video_sink), (gst_play_set_audio_sink),
(gst_play_set_visualization): Add some safety checks in set_ methods
and state_change. This was throwing some ugly CRITICAL messages when
pipeline was getting disposed and casts were failing.