fix HAVE_SDL_AUDIO flag of CMakeLists.txt II.

CMakeLists.txt

@@ -981,12 +981,14 @@ if(SDL_AUDIO)
     file(GLOB DUMMYAUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/dummy/*.c)
     set(SOURCE_FILES ${SOURCE_FILES} ${DUMMYAUDIO_SOURCES})
     set(HAVE_DUMMYAUDIO TRUE)
+    set(HAVE_SDL_AUDIO TRUE)
   endif()
   if(SDL_DISKAUDIO)
     set(SDL_AUDIO_DRIVER_DISK 1)
     file(GLOB DISKAUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/disk/*.c)
     set(SOURCE_FILES ${SOURCE_FILES} ${DISKAUDIO_SOURCES})
     set(HAVE_DISKAUDIO TRUE)
+    set(HAVE_SDL_AUDIO TRUE)
   endif()
 endif()