Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/bzip2.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': 'harfbuzz-ng',
+      'target_name': 'bzip2',
       'type': 'none',
-      'cflags': [
-        '<!@(pkg-config --cflags harfbuzz)',
-      ],
       'direct_dependent_settings': {
-        'cflags': [
-          '<!@(pkg-config --cflags harfbuzz)',
+        'defines': [
+          'USE_SYSTEM_LIBBZ2',
         ],
       },
       'link_settings': {
-        'ldflags': [
-          '<!@(pkg-config --libs-only-L --libs-only-other harfbuzz)',
-        ],
         'libraries': [
-          '<!@(pkg-config --libs-only-l harfbuzz)',
+          '-lbz2',
         ],
       },
     },
   ],
 }