Use a different define to decide which CRC library to use.

courgette/crc.cc

-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Calculate Crc by calling CRC method in LZMA SDK
-
 #include "courgette/crc.h"
 
+#ifdef COURGETTE_USE_CRC_LIB
+#  include "zlib.h"
+#else
 extern "C" {
-#include "third_party/lzma_sdk/7zCrc.h"
+#  include "third_party/lzma_sdk/7zCrc.h"
 }
+#endif
+
+#include "base/basictypes.h"
 
 namespace courgette {
 
 uint32 CalculateCrc(const uint8* buffer, size_t size) {
+  uint32 crc;
+
+#ifdef COURGETTE_USE_CRC_LIB
+  // Calculate Crc by calling CRC method in zlib
+  crc = crc32(0, buffer, size);
+#else
+  // Calculate Crc by calling CRC method in LZMA SDK
   CrcGenerateTable();
-  uint32 crc = 0xffffffffL;
-  crc = ~CrcCalc(buffer, size);
-  return crc;
+  crc = CrcCalc(buffer, size);
+#endif
+
+  return ~crc;
 }
 
 }  // namespace