only doing callback stuff when USE_THREADS is defined

src/Mixer/Channels.xs

@@ -22,6 +22,7 @@ static int sdl_perl_use_smpeg_audio = 0;
 #endif
 #endif
 
+#ifdef USE_THREADS
 static SV * cb = (SV*)NULL;
 
 void callback(int channel)
@@ -42,6 +43,7 @@ void callback(int channel)
 	FREETMPS;
 	LEAVE;
 }
+#endif
 
 MODULE = SDL::Mixer::Channels 	PACKAGE = SDL::Mixer::Channels    PREFIX = mixchan_
 
@@ -156,19 +158,30 @@ mixchan_fade_out_channel ( which, ms )
 	OUTPUT:
 		RETVAL
 
+#ifdef USE_THREADS
+
 void
 mixchan_channel_finished( fn )
 	SV* fn
 	CODE:
-		parent_perl = PERL_GET_CONTEXT;
-
 		if (cb == (SV*)NULL)
-            cb = newSVsv(fn);
+			cb = newSVsv(fn);
         else
-            SvSetSV(cb, fn);
-
+			SvSetSV(cb, fn);
+
+		parent_perl = PERL_GET_CONTEXT;
 		Mix_ChannelFinished(&callback);
 
+#else
+
+void
+mixchan_channel_finished( fn )
+	SV* fn
+	CODE:
+		warn("Perl need to be compiled with 'useithreads' for SDL::Mixer::Channels::channel_finished( cb )");
+
+#endif
+
 int
 mixchan_playing( channel )
 	int channel