Fixed building SDL applications with Visual Studio and the clang toolset

Also fixed building 64-bit SDL with clang. 32-bit doesn't build because of the inline assembly for C runtime support.

include/SDL_atomic.h

@@ -118,7 +118,7 @@ extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
  * The compiler barrier prevents the compiler from reordering
  * reads and writes to globally visible variables across the call.
  */
-#if defined(_MSC_VER) && (_MSC_VER > 1200)
+#if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
 void _ReadWriteBarrier(void);
 #pragma intrinsic(_ReadWriteBarrier)
 #define SDL_CompilerBarrier()   _ReadWriteBarrier()

include/SDL_config_windows.h

@@ -157,7 +157,7 @@ typedef unsigned int uintptr_t;
 /* Enable various audio drivers */
 #define SDL_AUDIO_DRIVER_WASAPI 1
 #define SDL_AUDIO_DRIVER_DSOUND 1
-#define SDL_AUDIO_DRIVER_XAUDIO2    1
+#define SDL_AUDIO_DRIVER_XAUDIO2    0
 #define SDL_AUDIO_DRIVER_WINMM  1
 #define SDL_AUDIO_DRIVER_DISK   1
 #define SDL_AUDIO_DRIVER_DUMMY  1

include/SDL_cpuinfo.h

@@ -33,6 +33,12 @@
 /* Need to do this here because intrin.h has C++ code in it */
 /* Visual Studio 2005 has a bug where intrin.h conflicts with winnt.h */
 #if defined(_MSC_VER) && (_MSC_VER >= 1500) && (defined(_M_IX86) || defined(_M_X64))
+#ifdef __clang__
+/* Many of the intrinsics SDL uses are not implemented by clang with Visual Studio */
+#undef __MMX__
+#undef __SSE__
+#undef __SSE2__
+#else
 #include <intrin.h>
 #ifndef _WIN64
 #define __MMX__
@@ -40,6 +46,7 @@
 #endif
 #define __SSE__
 #define __SSE2__
+#endif /* __clang__ */
 #elif defined(__MINGW64_VERSION_MAJOR)
 #include <intrin.h>
 #else