Use relative path to cygwin on Windows for the runtime

vm/vm.gypi

 # Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 {
   'variables': {
     'corelib_in_cc_file': 'corelib_in.cc',
     'corelib_cc_file': '<(SHARED_INTERMEDIATE_DIR)/corelib_gen.cc',
     'corelib_impl_in_cc_file': 'corelib_impl_in.cc',
     'corelib_impl_cc_file': '<(SHARED_INTERMEDIATE_DIR)/corelib_impl_gen.cc',
     'snapshot_test_dat_file': '<(SHARED_INTERMEDIATE_DIR)/snapshot_test.dat',
     'snapshot_test_in_dat_file': 'snapshot_test_in.dat',
     'snapshot_test_dart_file': 'snapshot_test.dart',
+    'cygwin_dir': '../../third_party/cygwin',
   },
   'targets': [
     {
       'target_name': 'libdart_vm',
       'type': 'static_library',
       'includes': [
         'vm_sources.gypi',
       ],
       'sources/': [
         # Exclude all _test.[cc|h] files.
@@ skipping 58 matching lines @@
       ],
       'sources': [
         '../vm/dart_api_impl.cc',
       ],
     },
     {
       'target_name': 'generate_corelib_cc_file',
       'type': 'none',
       'conditions': [
         ['OS=="win"', {
-          'msvs_cygwin_dirs': ['<(DEPTH)/third_party/cygwin'],
+          'msvs_cygwin_dirs': ['<(cygwin_dir)'],
         }],
       ],
       'includes': [
         # Load the runtime implementation sources.
         '../lib/lib_sources.gypi',
         # Load the shared core library sources.
         '../../corelib/src/corelib_sources.gypi',
       ],
       'sources/': [
         # Exclude all .[cc|h] files.
@@ skipping 22 matching lines @@
           ],
           'message': 'Generating ''<(corelib_cc_file)'' file.'
         },
       ]
     },
     {
       'target_name': 'generate_corelib_impl_cc_file',
       'type': 'none',
       'conditions': [
         ['OS=="win"', {
-          'msvs_cygwin_dirs': ['<(DEPTH)/third_party/cygwin'],
+          'msvs_cygwin_dirs': ['<(cygwin_dir)'],
         }],
       ],
       'includes': [
         # Load the runtime implementation sources.
         '../lib/lib_impl_sources.gypi',
         # Load the shared core library sources.
         '../../corelib/src/implementation/corelib_impl_sources.gypi',
       ],
       'sources/': [
         # Exclude all .[cc|h] files.
@@ skipping 22 matching lines @@
           ],
           'message': 'Generating ''<(corelib_impl_cc_file)'' file.'
         },
       ]
     },
     {
       'target_name': 'generate_snapshot_test_dat_file',
       'type': 'none',
       'conditions': [
         ['OS=="win"', {
-          'msvs_cygwin_dirs': ['<(DEPTH)/third_party/cygwin'],
+          'msvs_cygwin_dirs': ['<(cygwin_dir)'],
         }],
       ],
       'actions': [
         {
           'action_name': 'generate_snapshot_test_dat',
           'inputs': [
             '../tools/create_string_literal.py',
             '<(snapshot_test_in_dat_file)',
             '<(snapshot_test_dart_file)',
           ],
           'outputs': [
             '<(snapshot_test_dat_file)',
           ],
           'action': [
             'python',
             'tools/create_string_literal.py',
             '--output', '<(snapshot_test_dat_file)',
             '--input_cc', '<(snapshot_test_in_dat_file)',
             '<(snapshot_test_dart_file)',
           ],
           'message': 'Generating ''<(snapshot_test_dat_file)'' file.'
         },
       ]
     },
   ]
 }