Add explicit Release configuration to standalone.gypi to appease Xcode.

build/standalone.gypi

 # 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
@@ skipping 69 matching lines @@
     'armv7%': 1,
     'arm_neon%': 0,
     'arm_fpu%': 'vfpv3',
   },
   'target_defaults': {
     'default_configuration': 'Debug',
     'configurations': {
       'Debug': {
         'cflags': [ '-g', '-O0' ],
       },
+      'Release': {
+        # Xcode insists on this empty entry.
+      },
     },
   },
   'conditions': [
     ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris" \
        or OS=="netbsd"', {
       'target_defaults': {
         'cflags': [ '-Wall', '<(werror)', '-W', '-Wno-unused-parameter',
                     '-Wnon-virtual-dtor', '-pthread', '-fno-rtti',
                     '-fno-exceptions', '-pedantic' ],
         'ldflags': [ '-pthread', ],
@@ skipping 112 matching lines @@
         },
         'target_conditions': [
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
           }],
         ],  # target_conditions
       },  # target_defaults
     }],  # OS=="mac"
   ],
 }