Linux: add GYP flag to build with system libbz2.

chrome/common/bzip2_unittest.cc

 // Copyright (c) 2006-2008 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.
 
+#if defined(USE_SYSTEM_LIBBZ2)
+#include <bzlib.h>
+#else
+#include "third_party/bzip2/bzlib.h"
+#endif
+
 #include "base/basictypes.h"
 #include "testing/gtest/include/gtest/gtest.h"
-#include "third_party/bzip2/bzlib.h"
 
 namespace {
   class Bzip2Test : public testing::Test {
   };
 };
 
 // This test does a simple round trip to test that the bzip2 library is
 // present and working.
 TEST(Bzip2Test, Roundtrip) {
   char input[] = "Test Data, More Test Data, Even More Data of Test";
@@ skipping 40 matching lines @@
   do {
     result = BZ2_bzDecompress(&stream);
   } while (result == BZ_OK);
   ASSERT_EQ(result, BZ_STREAM_END);
   result = BZ2_bzDecompressEnd(&stream);
   ASSERT_EQ(result, BZ_OK);
 
   EXPECT_EQ(arraysize(input) - 1, stream.total_out_lo32);
   EXPECT_STREQ(input, output2);
 }