Add some basic infrastructure for Core Audio.

tests/mal_test_0.c

@@ -25,6 +25,7 @@ mal_backend g_Backends[] = {
     mal_backend_wasapi,
     mal_backend_dsound,
     mal_backend_winmm,
+    mal_backend_coreaudio,
     mal_backend_oss,
     mal_backend_pulseaudio,
     mal_backend_alsa,
@@ -2525,4 +2526,4 @@ int main(int argc, char** argv)
     #if defined(__GNUC__)
         #pragma GCC diagnostic pop
     #endif
-#endif
+#endif