Linux: add GYP flag to build with system libbz2.

net/base/bzip2_filter.h

 // 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.
 //
 // BZip2Filter applies bzip2 content encoding/decoding to a datastream.
 // Since it is a new feature, and no specification said what 's bzip2 content
 // composed of  in http protocol.  So I assume with bzip2 encoding the content
 // is full format, which means the content should carry complete bzip2 head,
 // such as inlcude magic number 1(BZh), block size bit, magic number 2(0x31,
 // 0x41, 0x59, 0x26, 0xx53, 0x59)
 // Maybe need to inserts a bzlib2 header to the data stream before calling
 // decompression functionality, but at now I do not meet this sort of real
 // scenarios. So let's see the further requests.
 //
 // This BZip2Filter internally uses third_party/bzip2 library to do decoding.
 //
 // BZip2Filter is also a subclass of Filter. See the latter's header file
 // filter.h for sample usage.
 
 #ifndef NET_BASE_BZIP2_FILTER_H_
 #define NET_BASE_BZIP2_FILTER_H_
 
+#if defined(USE_SYSTEM_LIBBZ2)
+#include <bzlib.h>
+#else
+#include "third_party/bzip2/bzlib.h"
+#endif
+
 #include "base/scoped_ptr.h"
 #include "net/base/filter.h"
-#include "third_party/bzip2/bzlib.h"
 
 class BZip2Filter : public Filter {
  public:
   explicit BZip2Filter(const FilterContext& filter_context);
 
   virtual ~BZip2Filter();
 
   // Initializes filter decoding mode and internal control blocks.
   // Parameter use_small_memory specifies whether use small memory
   // to decompresss data. If small is nonzero, the bzip2 library will
@@ skipping 38 matching lines @@
 
   // The control block of bzip which actually does the decoding.
   // This data structure is initialized by InitDecoding and updated in
   // ReadFilteredData.
   scoped_ptr<bz_stream> bzip2_data_stream_;
 
   DISALLOW_COPY_AND_ASSIGN(BZip2Filter);
 };
 
 #endif  // NET_BASE_BZIP2_FILTER_H_