Add support for Mac OS X 64bit builds with GYP

build/gyp_v8

 #!/usr/bin/python
 #
-# Copyright 2010 the V8 project authors. All rights reserved.
+# Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
 #       with the distribution.
@@ skipping 135 matching lines @@
   # to enfore syntax checking.
   syntax_check = os.environ.get('V8_GYP_SYNTAX_CHECK')
   if syntax_check and int(syntax_check):
     args.append('--check')
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Generate for the architectures supported on the given platform.
   gyp_args = list(args)
-  gyp_args.append('-Dtarget_arch=ia32')
+  target_arch = None
+  for p in gyp_args:
+    if p.find('-Dtarget_arch=') == 0:
+      target_arch = p
+  if target_arch is None:
+    gyp_args.append('-Dtarget_arch=ia32')
   if utils.GuessOS() == 'linux':
     gyp_args.append('-S-ia32')
   run_gyp(gyp_args)
 
   if utils.GuessOS() == 'linux':
     gyp_args = list(args)
     gyp_args.append('-Dtarget_arch=x64')
     gyp_args.append('-S-x64')
     run_gyp(gyp_args)
 
     gyp_args = list(args)
     gyp_args.append('-I' + v8_root + '/build/armu.gypi')
     gyp_args.append('-S-armu')
     run_gyp(gyp_args)
 
     gyp_args = list(args)
     gyp_args.append('-I' + v8_root + '/build/mipsu.gypi')
     gyp_args.append('-S-mipsu')
     run_gyp(gyp_args)