configure.ac: In the output at the end, don't show the first plugin on the same line as "Core plug-ins, always built:".

Original commit message from CVS:
* configure.ac:
In the output at the end, don't show the first plugin on the same
line as "Core plug-ins, always built:".
Indent the output as for other plugin categories
* gst/videotestsrc/gstvideotestsrc.c: (gst_videotestsrc_create):
#define that can be used to not use peer buffer_alloc functions for
test purposes.
* sys/ximage/ximagesink.c: (gst_ximage_buffer_init),
(gst_ximage_buffer_get_type), (gst_ximagesink_ximage_new),
(gst_ximagesink_show_frame):
* sys/xvimage/xvimagesink.c: (gst_xvimage_buffer_init),
(gst_xvimage_buffer_get_type), (gst_xvimagesink_setcaps),
(gst_xvimagesink_show_frame):
Error case handling fixes. gst-launch fakesrc ! x[v]imagesink now
fails gracefully instead of XError aborting or deadlocking.