Tweak -mssse3 on Mac a little.

gyp/opts.gyp

Index: gyp/opts.gyp
diff --git a/gyp/opts.gyp b/gyp/opts.gyp
index 8aa5e0f6bd1c7b1d5048a5278fd295b5b988e01b..a6436899bfbb837ae28baedab7f4c87e1c45d05a 100644
--- a/gyp/opts.gyp
+++ b/gyp/opts.gyp
@@ -124,12 +124,10 @@
             '-mssse3',
           ],
         }],
-        # TODO(epoger): the following will enable SSSE3 on Macs, but it will
-        # break once we set OTHER_CFLAGS anywhere else (the first setting will
-        # be replaced, not added to)
+        # This is a little redundant, as common_conditions enables SSSE3 globally on Macs.
         [ 'skia_os in ["mac"]', {
           'xcode_settings': {
-            'OTHER_CFLAGS': ['-mssse3',],
+            'OTHER_CPLUSPLUSFLAGS': ['-mssse3',],

[epoger, 2013/08/06 15:38:32]

I don't understand how this is not susceptible to the same problem as
OTHER_CFLAGS.  If OTHER_CPLUSPLUSFLAGS has been set elsewhere, this will nuke
whatever settings were made to it...

Adding Ben Wagner as a reviewer, because I think he has a better understanding
of the issues than I do.  Ben, any clever ideas here?

           },
         }],
         [ 'skia_arch_type == "x86"', {