Fix usage of SK_BUILD_* defines.

Since we just 'define' them, but not attribute anything to them, like
'1' for example, cpp expands it to nothing and that breaks the "#if"
clauses.

To fix that, uses "#if defined(...)" which will correctly check if your
macro name was defined or not.

BUG=skia:2850
TEST=make most
R=robertphillips@google.com

Review URL: https://codereview.chromium.org/628763005
diff --git a/tools/skpdiff/skpdiff_util.h b/tools/skpdiff/skpdiff_util.h
index 8750bf6..f626ac9 100644
--- a/tools/skpdiff/skpdiff_util.h
+++ b/tools/skpdiff/skpdiff_util.h
@@ -12,7 +12,7 @@
 #include "SkTArray.h"
 
 #if SK_SUPPORT_OPENCL
-#if SK_BUILD_FOR_MAC
+#if defined(SK_BUILD_FOR_MAC)
 #   include <OpenCL/cl.h>
 #else
 #   include <CL/cl.h>