Select GN build with an environment variable

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, 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.
 #
 
 import optparse
 import os
 import re
@@ skipping 26 matching lines @@
 usage = """\
 usage: %%prog [options] [targets]
 
 This script runs 'make' in the *current* directory. So, run it from
 the Dart repo root,
 
   %s ,
 
 unless you really intend to use a non-default Makefile.""" % DART_ROOT
 
+DART_USE_GN = "DART_USE_GN"
+
+
+def use_gn():
+  return DART_USE_GN in os.environ
+
+
 def BuildOptions():
   result = optparse.OptionParser(usage=usage)
   result.add_option("-m", "--mode",
       help='Build variants (comma-separated).',
       metavar='[all,debug,release,product]',
       default='debug')
   result.add_option("-v", "--verbose",
       help='Verbose output.',
       default=False, action="store_true")
   result.add_option("-a", "--arch",
@@ skipping 15 matching lines @@
   (vs_directory, vs_executable) = utils.GuessVisualStudioPath()
   result.add_option("--devenv",
       help='Path containing devenv.com on Windows',
       default=vs_directory)
   result.add_option("--executable",
       help='Name of the devenv.com/msbuild executable on Windows (varies for '
            'different versions of Visual Studio)',
       default=vs_executable)
   result.add_option("--gn",
       help='Build with GN/Ninja',
-      default=False,
+      default=use_gn(),
       action='store_true')
   return result
 
 
 def ProcessOsOption(os_name):
   if os_name == 'host':
     return HOST_OS
   return os_name
 
 
@@ skipping 496 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())