make OpenCL optional for skpdiff

experimental/skpdiff/diff_pixels.cl

-/*
- * Copyright 2013 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#pragma OPENCL_EXTENSION cl_khr_global_int32_base_atomics
-
-const sampler_t gInSampler = CLK_NORMALIZED_COORDS_FALSE |
-                             CLK_ADDRESS_CLAMP_TO_EDGE   |
-                             CLK_FILTER_NEAREST;
-
-__kernel void diff(read_only image2d_t baseline, read_only image2d_t test,
-                   __global int* result, __global int2* poi) {
-    int2 coord = (int2)(get_global_id(0), get_global_id(1));
-    uint4 baselinePixel = read_imageui(baseline, gInSampler, coord);
-    uint4 testPixel = read_imageui(test, gInSampler, coord);
-    int4 pixelCompare = baselinePixel == testPixel;
-    if (baselinePixel.x != testPixel.x ||
-        baselinePixel.y != testPixel.y ||
-        baselinePixel.z != testPixel.z ||
-        baselinePixel.w != testPixel.w) {
-
-        int poiIndex = atomic_inc(result);
-        poi[poiIndex] = coord;
-    }
-}