resolve implicit declarations and enforce warnings as errors

include/m4a.h

@@ -0,0 +1,18 @@
+#ifndef GUARD_M4A_H
+#define GUARD_M4A_H
+
+#include "gba/m4a_internal.h"
+
+void m4aSoundVSync(void);
+
+void m4aSoundInit(void);
+void m4aSoundMain(void);
+void m4aSongNumStart(u16);
+void m4aSongNumStop(u16 n);
+void m4aMPlayContinue(struct MusicPlayerInfo *mplayInfo);
+void m4aMPlayFadeOut(struct MusicPlayerInfo *mplayInfo, u16 speed);
+void m4aMPlayFadeOutTemporarily(struct MusicPlayerInfo *mplayInfo, u16 speed);
+void m4aMPlayFadeIn(struct MusicPlayerInfo *mplayInfo, u16 speed);
+void m4aMPlayImmInit(struct MusicPlayerInfo *mplayInfo);
+
+#endif //GUARD_M4A_H