Add AVX flag for Windows GN builds

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/arm.gni")
 import("//build/config/android/config.gni")
 import("//third_party/libvpx/libvpx_srcs.gni")
 import("//third_party/yasm/yasm_assemble.gni")
 
 if (is_posix && !is_mac) {
@@ skipping 113 matching lines @@
     sources = libvpx_srcs_x86_sse4_1
   } else if (cpu_arch == "x64") {
     sources = libvpx_srcs_x86_64_sse4_1
   }
 }
 
 static_library("libvpx_intrinsics_avx2") {
   configs += [ ":libvpx_config" ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
-  if (!is_win) {
+  if (is_win) {
+    cflags = [ "/arch:AVX" ]

[Tom Finegan, 2015/01/22 18:43:49]

msdn vs2013 docs[1] say both are supported. I think vs2013 is the minimum
required compiler for windows chromium development, so AVX2 should be OK (and is
probably what you want since we're using -mavx2 everywhere else).

[1] https://msdn.microsoft.com/en-us/library/jj620901.aspx

+  } else {
     cflags = [ "-mavx2" ]
   }
   if (cpu_arch == "x86") {
     sources = libvpx_srcs_x86_avx2
   } else if (cpu_arch == "x64") {
     sources = libvpx_srcs_x86_64_avx2
   }
 }
 
 if (cpu_arch_full == "arm-neon-cpu-detect") {
@@ skipping 97 matching lines @@
   }
   if (is_android) {
     deps += [ "//third_party/android_tools:cpu_features" ]
   }
   if (cpu_arch == "arm") {
     deps += [ ":libvpx_assembly_arm" ]
   }
 
   public_configs = [ ":libvpx_external_config" ]
 }