Attempt to let skia's SSE files build with clang-cl.

skia/skia_library_opts.gyp

Index: skia/skia_library_opts.gyp
diff --git a/skia/skia_library_opts.gyp b/skia/skia_library_opts.gyp
index cc002923b1b7daff78ac44152b2ea471272c6f4c..821168bc0326bbb75b1c8bed94b0591710c18586 100644
--- a/skia/skia_library_opts.gyp
+++ b/skia/skia_library_opts.gyp
@@ -177,6 +177,13 @@
             'GCC_ENABLE_SUPPLEMENTAL_SSE3_INSTRUCTIONS': 'YES',
           },
         }],
+        [ 'OS == "win" and clang == 1', {
+          # cl.exe's /arch flag doesn't have a setting for SSSE3, and cl.exe
+          # doesn't need it for intrinsics. clang-cl does need it, though.
+          'msvs_settings': {
+            'VCCLCompilerTool': { 'AdditionalOptions': [ '-mssse3' ] },
+          },
+        }],
         [ 'OS == "win"', {
           'include_dirs': [
             'config/win',
@@ -226,6 +233,13 @@
             'GCC_ENABLE_SSE41_EXTENSIONS': 'YES',
           },
         }],
+        [ 'OS == "win" and clang == 1', {
+          # cl.exe's /arch flag doesn't have a setting for SSE4.1, and cl.exe
+          # doesn't need it for intrinsics. clang-cl does need it, though.
+          'msvs_settings': {
+            'VCCLCompilerTool': { 'AdditionalOptions': [ '-msse4.1' ] },
+          },
+        }],
         [ 'OS == "win"', {
           'defines' : [
             'SK_CPU_SSE_LEVEL=41'