Don't use automatic toolchain if GYP_MSVS_VERSION overridden to another version

build/gyp_chromium

 #!/usr/bin/env python
 
 # Copyright (c) 2012 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.
 
 # This script is wrapper for Chromium that adds some support for how GYP
 # is invoked by Chromium beyond what can be done in the gclient hooks.
 
 import glob
@@ skipping 423 matching lines @@
     os.environ['GYP_GENERATORS'] = 'ninja'
   if sys.platform.startswith('win') and not os.environ.get('GYP_GENERATORS'):
     os.environ['GYP_GENERATORS'] = 'ninja'
   elif sys.platform == 'darwin' and not os.environ.get('GYP_GENERATORS') and \
       not 'OS=ios' in os.environ.get('GYP_DEFINES', []):
     os.environ['GYP_GENERATORS'] = 'ninja'
 
   # If on windows, and the automatic toolchain has been installed by
   # depot_tools, then use it.
   vs2013_runtime_dll_dirs = None
-  if sys.platform in ('win32', 'cygwin'):
+  # If MSVS_VERSION is explicitly specified to be something other than 2013,
+  # don't use the automatic toolchain, as it currently only supports VS2013.
+  msvs_version = os.environ.get('GYP_MSVS_VERSION', '2013')
+  if sys.platform in ('win32', 'cygwin') and msvs_version.startswith('2013'):
     import find_depot_tools
     depot_tools_path = find_depot_tools.add_depot_tools_to_path()
     toolchain = os.path.normpath(os.path.join(
         depot_tools_path, 'win_toolchain', 'vs2013_files'))
     version_file = os.path.join(toolchain, '.version')
     if os.path.isdir(toolchain) and os.path.isfile(version_file):
       os.environ['GYP_MSVS_OVERRIDE_PATH'] = toolchain
       with open(version_file, 'r') as f:
         version_is_pro = f.read().strip() == 'pro'
       vs2013_runtime_dll_dirs = (os.path.join(toolchain, 'sys32'),
@@ skipping 42 matching lines @@
   # from above are picked up.
   print 'Running build/landmines.py...'
   subprocess.check_call(
       [sys.executable, os.path.join(script_dir, 'landmines.py')])
 
   if vs2013_runtime_dll_dirs:
     CopyVsRuntimeDlls(os.path.join(chrome_src, GetOutputDirectory()),
                       vs2013_runtime_dll_dirs)
 
   sys.exit(gyp_rc)