decoder/wildmidi: support version 0.2.3

In libwildmidi 0.2.3, the function WildMidi_SampledSeek() was removed,
without changing the SO name.  This patch adds an autoconf check for
that function.  Fall back to WildMidi_FastSeek() if
WildMidi_SampledSeek() is not available anymore.

src/decoder/wildmidi_plugin.c

@@ -99,7 +99,11 @@ wildmidi_file_decode(struct decoder *decoder, const char *path_fs)
 			unsigned long seek_where = WILDMIDI_SAMPLE_RATE *
 				decoder_seek_where(decoder);
 
+#ifdef HAVE_WILDMIDI_SAMPLED_SEEK
 			WildMidi_SampledSeek(wm, &seek_where);
+#else
+			WildMidi_FastSeek(wm, &seek_where);
+#endif
 			decoder_command_finished(decoder);
 			cmd = DECODE_COMMAND_NONE;
 		}