configure.ac: Use AC_SYS_LARGEFILE, which will set _FILE_OFFSET_BITS=64 and _LARGEFIL...

configure.ac: Use AC_SYS_LARGEFILE, which will set _FILE_OFFSET_BITS=64 and _LARGEFILE_SOURCE in config.h as required...

Original commit message from CVS:
* configure.ac:
Use AC_SYS_LARGEFILE, which will set _FILE_OFFSET_BITS=64
and _LARGEFILE_SOURCE in config.h as required. Do not
export those flags in our .pc files any longer (#142209).
Remove unused GST_DISABLE_OMEGA_COTHREADS stuff.
* gst/elements/gstfilesink.c: (gst_file_sink_class_init),
(gst_file_sink_do_seek), (gst_file_sink_event),
(gst_file_sink_get_current_offset), (gst_file_sink_render):
Redo seek/tell calls with large file support in mind; add some
debugging messages; add log message that tells us when large
file support is unavailable or not enabled for some reason.
* gst/elements/gstfilesrc.c: (gst_file_src_class_init):
Add log message that tells us when large file support
is unavailable or not enabled for some reason.