Add support for system snappy.

build/linux/unbundle/snappy.gyp

 # Copyright 2013 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.
 
 {
   'targets': [
     {
-      'target_name': 'libevent',
+      'target_name': 'snappy',
       'type': 'none',
-      'toolsets': ['host', 'target'],
       'variables': {
-        'headers_root_path': '.',
+        'headers_root_path': 'src',
         'header_filenames': [
-          'event.h',
+          'snappy-c.h',
+          'snappy-sinksource.h',
+          'snappy-stubs-public.h',
+          'snappy.h',
         ],
       },
       'includes': [
         '../../build/shim_headers.gypi',
       ],
       'link_settings': {
         'libraries': [
-          '-levent',
+          '-lsnappy',
         ],
       },
-    }
+    },
   ],
 }