Use <(DEPTH) instead of fixed relative path to avoid issues

third_party/libc++/libc++.gyp

 # Copyright 2015 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.
 
 {
   'variables': {
-    'libcxx_root': '../../buildtools/third_party/libc++',
+    'libcxx_root': '<(DEPTH)/buildtools/third_party/libc++',
   },
   'targets': [
     {
       'target_name': 'libcxx_proxy',
       'type': 'none',
       'toolsets': ['host', 'target'],
       'dependencies=': [
         'libc++',
       ],
       # Do not add dependency on libc++.so to dependents of this target. We
@@ skipping 99 matching lines @@
       'libraries': [
         '-lc',
         '-lgcc_s',
         '-lm',
         '-lpthread',
         '-lrt',
       ],
     },
   ]
 }