Add libopenspc-based SPC decoder element (#348220).

Add libopenspc-based SPC decoder element (#348220).

Original commit message from CVS:
Patch by: Chris Lee  <clee at kde org>
* configure.ac:
* ext/Makefile.am:
* ext/spc/Makefile.am:
* ext/spc/gstspc.c: (spc_negotiate), (gst_spc_dec_base_init),
(gst_spc_dec_class_init), (gst_spc_dec_init), (gst_spc_dec_chain),
(gst_spc_dec_sink_event), (gst_spc_dec_src_event),
(gst_spc_dec_src_query), (spc_play), (spc_setup),
(gst_spc_dec_change_state), (plugin_init):
* ext/spc/gstspc.h:
Add libopenspc-based SPC decoder element (#348220).