Support joined -D flags for gn.

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 103 matching lines @@
         supp_items += [(FormatKeyForGN(v), str(variables[v]))]
 
   # GYP defines from the environment.
   env_items = ProcessGypDefinesItems(
       shlex.split(os.environ.get('GYP_DEFINES', '')))
 
   # GYP defines from the command line. We can't use optparse since we want
   # to ignore all arguments other than "-D".
   cmdline_input_items = []
   for i in range(len(sys.argv))[1:]:
-    if sys.argv[i] == '-D' and i + 1 < len(sys.argv):
-      cmdline_input_items += [sys.argv[i + 1]]
+    if sys.argv[i].startswith('-D'):
+      if sys.argv[i] == '-D' and i + 1 < len(sys.argv):
+        cmdline_input_items += [sys.argv[i + 1]]
+      elif len(sys.argv[i]) > 2:
+        cmdline_input_items += [sys.argv[i][2:]]
   cmdline_items = ProcessGypDefinesItems(cmdline_input_items)
 
   return dict(supp_items + env_items + cmdline_items)
 
 def GetOutputDirectory():
   """Returns the output directory that GYP will use."""
   # GYP generator flags from the command line. We can't use optparse since we
   # want to ignore all arguments other than "-G".
   needle = '-Goutput_dir='
   cmdline_input_items = []
@@ skipping 287 matching lines @@
   args.extend(
       ['-I' + i for i in additional_include_files(supplemental_includes, args)])
 
   args.extend(['-D', 'gyp_output_dir=' + GetOutputDirectory()])
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Off we go...
   sys.exit(gyp.main(args))