make OpenCL optional for skpdiff

- add CPU based different pixels metric

R=djsollen@google.com

Review URL: https://codereview.chromium.org/19374006

git-svn-id: http://skia.googlecode.com/svn/trunk@10127 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/experimental/skpdiff/skpdiff_util.cpp b/experimental/skpdiff/skpdiff_util.cpp
index 3a36a12..1cf0261 100644
--- a/experimental/skpdiff/skpdiff_util.cpp
+++ b/experimental/skpdiff/skpdiff_util.cpp
@@ -11,6 +11,7 @@
 #include "SkOSFile.h"
 #include "skpdiff_util.h"
 
+#if SK_SUPPORT_OPENCL
 const char* cl_error_to_string(cl_int err) {
     switch (err) {
         case CL_SUCCESS:                         return "CL_SUCCESS";
@@ -63,6 +64,7 @@
     }
     return "UNKNOWN";
 }
+#endif
 
 
 double get_seconds() {