more sparse cleanups

* less-commonly compiled things like ao/mvp outputs
* Adding -Wno-transparent-union to SPARSE_FLAGS makes it check
inside decode.c, directory.c, player.c, and sig_handlers.c
* remove unused variables leftover from the master process
in sig_handlers.c

git-svn-id: https://svn.musicpd.org/mpd/trunk@4598 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/decode.c

@@ -680,7 +680,7 @@ void decodeParent(PlayerControl * pc, DecoderControl * dc, OutputBuffer * cb)
  * child process does decoding
  * parent process does playing audio
  */
-void decode()
+void decode(void)
 {
 	OutputBuffer *cb;
 	PlayerControl *pc;