Use the correct AVX flags for Windows builds

libvpx_srcs_x86_64_intrinsics.gypi

Index: libvpx_srcs_x86_64_intrinsics.gypi
diff --git a/libvpx_srcs_x86_64_intrinsics.gypi b/libvpx_srcs_x86_64_intrinsics.gypi
index bd81bd9b03ea87d9d9131e4469e324eb73a5434d..a812e3aab8bf7171f28f74c512e0d0b0c361d4ad 100644
--- a/libvpx_srcs_x86_64_intrinsics.gypi
+++ b/libvpx_srcs_x86_64_intrinsics.gypi
@@ -66,7 +66,8 @@
       'conditions': [
         ['OS=="win" and clang==1', {
           # cl.exe's /arch flag doesn't have a setting for SSSE3/4, and cl.exe
-          # doesn't need it for intrinsics. clang-cl does need it, though.
+          # doesn't need it for intrinsics. There is an /arch flag for AVX2 but
+          # clang-cl needs its own in the form of -mOPT.
           'msvs_settings': {
             'VCCLCompilerTool': { 'AdditionalOptions': [ '-mssse3' ] },
           },
@@ -88,7 +89,8 @@
       'conditions': [
         ['OS=="win" and clang==1', {
           # cl.exe's /arch flag doesn't have a setting for SSSE3/4, and cl.exe
-          # doesn't need it for intrinsics. clang-cl does need it, though.
+          # doesn't need it for intrinsics. There is an /arch flag for AVX2 but
+          # clang-cl needs its own in the form of -mOPT.
           'msvs_settings': {
             'VCCLCompilerTool': { 'AdditionalOptions': [ '-msse4.1' ] },
           },
@@ -121,6 +123,16 @@
           'EnableEnhancedInstructionSet': '3', # /arch:AVX
         },
       },
+      'conditions': [
+        ['OS=="win" and clang==1', {
+          # cl.exe's /arch flag doesn't have a setting for SSSE3/4, and cl.exe
+          # doesn't need it for intrinsics. There is an /arch flag for AVX2 but
+          # clang-cl needs its own in the form of -mOPT.
+          'msvs_settings': {
+            'VCCLCompilerTool': { 'AdditionalOptions': [ '-mavx2' ] },
+          },
+        }],
+      ],
     },
   ],
 }