make OpenCL optional for skpdiff

gyp/common_conditions.gypi

 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
 #    'SK_SUPPORT_HINTING_SCALE_FACTOR',
   ],
   'conditions' : [
     [ 'skia_gpu == 1',
       {
         'defines': [
           'SK_SUPPORT_GPU=1',
         ],
       }, {
         'defines': [
           'SK_SUPPORT_GPU=0',
         ],
       },
     ],
-
+    [ 'skia_opencl == 1',
+      {
+        'defines': [
+          'SK_SUPPORT_OPENCL=1',
+        ],
+      }, {
+        'defines': [
+          'SK_SUPPORT_OPENCL=0',
+        ],
+      },
+    ],
     [ 'skia_os == "win"',
       {
         'defines': [
           'SK_BUILD_FOR_WIN32',
           'SK_IGNORE_STDINT_DOT_H',
           '_CRT_SECURE_NO_WARNINGS',
           'GR_GL_FUNCTION_TYPE=__stdcall',
         ],
         'msvs_cygwin_shell': 0,
         'msvs_settings': {
@@ skipping 423 matching lines @@
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }
 
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