Fix -Wundef warnings due to use of unguarded SDL_AUDIO_DRIVER_NETBSD

src/audio/SDL_audio.c

@@ -43,7 +43,7 @@ static const AudioBootStrap *const bootstrap[] = {
 #ifdef SDL_AUDIO_DRIVER_SNDIO
     &SNDIO_bootstrap,
 #endif
-#if SDL_AUDIO_DRIVER_NETBSD
+#ifdef SDL_AUDIO_DRIVER_NETBSD
     &NETBSDAUDIO_bootstrap,
 #endif
 #if SDL_AUDIO_DRIVER_WASAPI

src/audio/SDL_audiodev.c

@@ -22,7 +22,7 @@
 
 /* Get the name of the audio device we use for output */
 
-#if SDL_AUDIO_DRIVER_NETBSD || SDL_AUDIO_DRIVER_OSS || SDL_AUDIO_DRIVER_SUNAUDIO
+#if defined(SDL_AUDIO_DRIVER_NETBSD) || SDL_AUDIO_DRIVER_OSS || SDL_AUDIO_DRIVER_SUNAUDIO
 
 #include <fcntl.h>
 #include <sys/types.h>

src/audio/netbsd/SDL_netbsdaudio.c

@@ -20,7 +20,7 @@
 */
 #include "SDL_internal.h"
 
-#if SDL_AUDIO_DRIVER_NETBSD
+#if defined(SDL_AUDIO_DRIVER_NETBSD)
 
 /*
  * Driver for native NetBSD audio(4).