Make Alsa audio backend optional

src/audio.h

@@ -27,7 +27,9 @@
 
 extern const char* audio_device;
 
+#ifdef HAVE_ALSA
 extern AUDIO_RENDERER_CALLBACKS audio_callbacks_alsa;
+#endif
 #ifdef HAVE_SDL
 extern AUDIO_RENDERER_CALLBACKS audio_callbacks_sdl;
 #endif

src/platform.c

@@ -111,7 +111,9 @@ AUDIO_RENDERER_CALLBACKS* platform_get_audio(enum platform system) {
     if (audio_pulse_init())
       return &audio_callbacks_pulse;
     #endif
+    #ifdef HAVE_ALSA
     return &audio_callbacks_alsa;
+    #endif
   }
   return NULL;
 }