The only differences we have with the upstream tarball are marked with
`// -GODOT-` comments for clarity. The changes we currently have are
just some defines for cross-platform configuration, and could likely be
moved to the SCons buildsystem (ideally in a sub-environment to avoid
having RtAudio-specific defines pollute the whole build log).

One change for WinRT was not kept, if it is needed it should ideally be
integrated upstream:
```
 #if defined(__WINDOWS_DS__) || defined(__WINDOWS_ASIO__) || defined(__WINDOWS_WASAPI__)
-#ifdef WINRT_ENABLED
-  #define MUTEX_INITIALIZE(A) InitializeCriticalSectionEx(A, 0, 0)
-#else
   #define MUTEX_INITIALIZE(A) InitializeCriticalSection(A)
-#endif
```