Miscellaneous fixes for system libraries files:

build/linux/unbundle/libvpx.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': 'libvpx',
       'type': 'none',
       'direct_dependent_settings': {
         'cflags': [
           '<!@(pkg-config --cflags vpx)',
         ],
       },
       'variables': {
-        'headers_root_path': '<(libvpx_source)',
+        'headers_root_path': 'source/libvpx',
         'header_filenames': [
           'vpx/vpx_codec_impl_bottom.h',
           'vpx/vpx_image.h',
           'vpx/vpx_decoder.h',
           'vpx/vp8.h',
           'vpx/vpx_codec.h',
           'vpx/vpx_codec_impl_top.h',
           'vpx/vp8cx.h',
           'vpx/vpx_integer.h',
           'vpx/vp8dx.h',
           'vpx/vpx_encoder.h',
         ],
       },
       'includes': [
         '../../build/shim_headers.gypi',
       ],
       'link_settings': {
         'ldflags': [
           '<!@(pkg-config --libs-only-L --libs-only-other vpx)',
         ],
         'libraries': [
           '<!@(pkg-config --libs-only-l vpx)',
         ],
       },
     },
   ],
 }