Add a GYP flag to define SKNX_NO_SIMD when set.

gyp/common_conditions.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
@@ skipping 610 matching lines @@
         ],
       },
     ],
 
     [ 'skia_moz2d', {
       'defines': [
         # add flags here (e.g. SK_SUPPORT_LEGACY_...) needed by moz2d
       ],
     }],
 
+    [ 'sknx_no_simd', {
+      'defines': [ 'SKNX_NO_SIMD' ],
+    }],
+
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }