make OpenCL optional for skpdiff

experimental/skpdiff/skpdiff.gyp

 # GYP file to build skpdiff.
 #
 # To build on Linux:
 #  ./gyp_skia skpdiff.gyp && make skpdiff
 #
 {
   'targets': [
     {
       'target_name': 'skpdiff',
       'type': 'executable',
       'sources': [
         'main.cpp',
         'SkDiffContext.cpp',
         'SkImageDiffer.cpp',
-        'SkCLImageDiffer.cpp',
         'SkPMetric.cpp',
         'skpdiff_util.cpp',
         '../../tools/flags/SkCommandLineFlags.cpp',
       ],
       'include_dirs': [
         '../../tools/flags'
       ],
       'dependencies': [
         '../../gyp/skia_lib.gyp:skia_lib',
       ],
       'cflags': [
         '-O3',
       ],
-      'link_settings': {
-        'libraries': [
-          '-lOpenCL',
-        ],
-      },
+      'conditions': [
+        ['skia_opencl', {
+          'sources': [
+            'SkCLImageDiffer.cpp',
+            'SkDifferentPixelsMetric_opencl.cpp',
+          ],
+          'link_settings': {
+            'libraries': [
+              '-lOpenCL',
+            ],
+          },
+        }, {
+          'sources': [
+            'SkDifferentPixelsMetric_cpu.cpp',
+          ],
+        }],
+      ],
     },
   ],
-  'conditions': [
-  ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: