Only define SSE2 if necessary

BUG=224
TEST=compile with gcc 4.8 on msys
Review URL: https://webrtc-codereview.appspot.com/1411004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@684 16f28f9a-4ce2-e073-06de-1de4eb20be90

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 683
+Version: 684
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 683
+#define LIBYUV_VERSION 684
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT

util/ssim.cc

@@ -20,7 +20,7 @@ extern "C" {
 typedef unsigned int uint32;     // NOLINT
 typedef unsigned short uint16;   // NOLINT
 
-#if !defined(LIBYUV_DISABLE_X86) && \
+#if !defined(LIBYUV_DISABLE_X86) && !defined(__SSE2__) && \
   (defined(_M_X64) || (defined(_M_IX86_FP) && (_M_IX86_FP >= 2)))
 #define __SSE2__
 #endif