sunaudio: don't compile in the sun audio backend if sys/audioio.h is missing.

Newer Solaris-based releases, like OpenIndiana, don't include this, using
Open Sound System, or something layered over OSS like PulseAudio, instead.

Fixes #13405.

CMakeLists.txt

@@ -1470,10 +1470,14 @@ elseif(EMSCRIPTEN)
 elseif(UNIX AND NOT APPLE AND NOT RISCOS AND NOT HAIKU)
   if(SDL_AUDIO)
     if(SYSV5 OR SOLARIS OR HPUX)
-        set(SDL_AUDIO_DRIVER_SUNAUDIO 1)
-        file(GLOB SUN_AUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/sun/*.c)
-        list(APPEND SOURCE_FILES ${SUN_AUDIO_SOURCES})
-        set(HAVE_SDL_AUDIO TRUE)
+        # Newer Solaris-based systems, like OpenIndiana, don't have this interface anymore. Check for the header first.
+        check_include_file(sys/audioio HAVE_SYS_AUDIOIO_H)
+        if(HAVE_SYS_AUDIOIO_H)
+            set(SDL_AUDIO_DRIVER_SUNAUDIO 1)
+            file(GLOB SUN_AUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/sun/*.c)
+            list(APPEND SOURCE_FILES ${SUN_AUDIO_SOURCES})
+            set(HAVE_SDL_AUDIO TRUE)
+        endif()
     elseif(NETBSD)
         set(SDL_AUDIO_DRIVER_NETBSD 1)
         file(GLOB NETBSD_AUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/netbsd/*.c)