configure.ac: Check for stdio_ext.h for the filesink changes.

configure.ac: Check for stdio_ext.h for the filesink changes.

Original commit message from CVS:
Based on Patch by: Laurent Glayal <spglegle at yahoo dot fr>
* configure.ac:
Check for stdio_ext.h for the filesink changes.
* plugins/elements/gstfilesink.c: (buffer_mode_get_type),
(gst_file_sink_class_init), (gst_file_sink_init),
(gst_file_sink_dispose), (gst_file_sink_set_property),
(gst_file_sink_get_property), (gst_file_sink_open_file),
(gst_file_sink_close_file):
* plugins/elements/gstfilesink.h:
Add two properties to control the buffering mode and size.
API: GstFileSink::buffer-mode
API: GstFileSink::buffer-size