Simplify line editor choice in d8.

src/d8.gyp

 # 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 30 matching lines @@
       'include_dirs+': [
         '../src',
       ],
       'sources': [
         'd8.cc',
       ],
       'conditions': [
         [ 'console=="readline"', {
           'libraries': [ '-lreadline', ],
           'sources': [ 'd8-readline.cc' ],
+          'defines': ['USE_READLINE_CONSOLE'],
         }],
         [ 'component!="shared_library"', {
           'sources': [ 'd8-debug.cc', '<(SHARED_INTERMEDIATE_DIR)/d8-js.cc', ],
           'conditions': [
             [ 'want_separate_host_toolset==1', {
               'dependencies': [
                 'd8_js2c#host',
               ],
             }, {
               'dependencies': [
@@ skipping 43 matching lines @@
             '<@(_outputs)',
             'D8',
             'off',  # compress startup data
             '<@(js_files)'
           ],
         },
       ],
     }
   ],
 }