Linux: add GYP flag to build with system libbz2.

net/base/bzip2_filter_unittest.cc

Index: net/base/bzip2_filter_unittest.cc
diff --git a/net/base/bzip2_filter_unittest.cc b/net/base/bzip2_filter_unittest.cc
index 9b1eab5fa80a24ed1801ac8d79bc3c36fb7b4efb..7705ebbc73212b2ecc702f4eda05d4af6bd93eb5 100644
--- a/net/base/bzip2_filter_unittest.cc
+++ b/net/base/bzip2_filter_unittest.cc
@@ -5,6 +5,12 @@
 #include <fstream>
 #include <iostream>
 
+#if defined(USE_SYSTEM_LIBBZ2)
+#include <bzlib.h>
+#else
+#include "third_party/bzip2/bzlib.h"
+#endif
+
 #include "base/file_util.h"
 #include "base/path_service.h"
 #include "base/scoped_ptr.h"
@@ -13,7 +19,6 @@
 #include "net/base/io_buffer.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
-#include "third_party/bzip2/bzlib.h"
 
 namespace {