Use clang front end ifdef for AVX2 instead of llvm backend.

BUG=269
TESTED=clang++ -c source/row_posix.cc -I include/
R=ryanpetrie@google.com

Review URL: https://webrtc-codereview.appspot.com/2266004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@796 16f28f9a-4ce2-e073-06de-1de4eb20be90

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 795
+Version: 796
 License: BSD
 License File: LICENSE
 

include/libyuv/row.h

@@ -146,7 +146,7 @@ extern "C" {
 // Caveat: llvm 3.1 required, but does not provide a version.
 #if !defined(LIBYUV_DISABLE_X86) && \
   ((defined(_M_IX86) && defined(_MSC_VER) && _MSC_VER >= 1700) || \
-  defined(__native_client__) || defined(__llvm__))
+  defined(__native_client__) || defined(__clang__))
 // Effects:
 #define HAS_ARGBPOLYNOMIALROW_AVX2
 #endif

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 795
+#define LIBYUV_VERSION 796
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT