Port flacdec (seeking is still slow'ish).

Port flacdec (seeking is still slow'ish).

Original commit message from CVS:
* configure.ac:
* ext/Makefile.am:
* ext/flac/Makefile.am:
* ext/flac/gstflac.c: (plugin_init):
* ext/flac/gstflacdec.c: (flacdec_get_type), (gst_flacdec_init),
(gst_flacdec_update_metadata), (gst_flacdec_seek),
(gst_flacdec_tell), (gst_flacdec_length), (gst_flacdec_read),
(gst_flacdec_write), (gst_flacdec_loop),
(gst_flacdec_get_src_query_types), (gst_flacdec_src_query),
(gst_flacdec_src_event), (gst_flacdec_sink_activate),
(gst_flacdec_sink_activate_pull), (gst_flacdec_change_state):
* ext/flac/gstflacdec.h:
Port flacdec (seeking is still slow'ish).