Fix usage of SK_BUILD_* defines.

tools/skdiff.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef skdiff_DEFINED
 #define skdiff_DEFINED
 
 #include "SkBitmap.h"
 #include "SkColor.h"
 #include "SkColorPriv.h"
 #include "SkString.h"
 #include "SkTDArray.h"
 
-#if SK_BUILD_FOR_WIN32
+#if defined(SK_BUILD_FOR_WIN32)
     #define PATH_DIV_STR "\\"
     #define PATH_DIV_CHAR '\\'
 #else
     #define PATH_DIV_STR "/"
     #define PATH_DIV_CHAR '/'
 #endif
 
 #define MAX2(a,b) (((b) < (a)) ? (a) : (b))
 #define MAX3(a,b,c) (((b) < (a)) ? MAX2((a), (c)) : MAX2((b), (c)))
 
@@ skipping 235 matching lines @@
     return SkPackARGB32(0xFF, SkAbs32(dr), SkAbs32(dg), SkAbs32(db));
 }
 
 /** When finished, dr->fResult should have some value other than kUnknown_Result.
  *  Expects dr->fWhite.fBitmap and dr->fDifference.fBitmap to have the same bounds as
  *  dr->fBase.fBitmap and have a valid pixelref.
  */
 void compute_diff(DiffRecord* dr, DiffMetricProc diffFunction, const int colorThreshold);
 
 #endif