fix building SDL_audiotypecvt.c with gcc < 4.0

src/audio/SDL_audiotypecvt.c

@@ -509,6 +509,16 @@ SDL_Convert_U16_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format)
     }
 }
 
+#if defined(__GNUC__) && (__GNUC__ < 4)
+/* these were added as of gcc-4.0: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19418 */
+static inline __m128 _mm_castsi128_ps(__m128i __A) {
+  return (__m128) __A;
+}
+static inline __m128i _mm_castps_si128(__m128 __A) {
+  return (__m128i) __A;
+}
+#endif
+
 static void SDLCALL
 SDL_Convert_S32_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format)
 {