Fix compilation with recent MinGW64 versions.

src/flag-definitions.h

 // 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 602 matching lines @@
 
 // Regexp
 DEFINE_bool(regexp_optimization, true, "generate optimized regexp code")
 
 // Testing flags test/cctest/test-{flags,api,serialization}.cc
 DEFINE_bool(testing_bool_flag, true, "testing_bool_flag")
 DEFINE_int(testing_int_flag, 13, "testing_int_flag")
 DEFINE_float(testing_float_flag, 2.5, "float-flag")
 DEFINE_string(testing_string_flag, "Hello, world!", "string-flag")
 DEFINE_int(testing_prng_seed, 42, "Seed used for threading test randomness")
-#ifdef WIN32
+#ifdef _WIN32
 DEFINE_string(testing_serialization_file, "C:\\Windows\\Temp\\serdes",
               "file in which to testing_serialize heap")
 #else
 DEFINE_string(testing_serialization_file, "/tmp/serdes",
               "file in which to serialize heap")
 #endif
 
 // mksnapshot.cc
 DEFINE_string(extra_code, NULL, "A filename with extra code to be included in"
                   " the snapshot (mksnapshot only)")
@@ skipping 218 matching lines @@
 #undef DEFINE_ALIAS_int
 #undef DEFINE_ALIAS_string
 #undef DEFINE_ALIAS_float
 #undef DEFINE_ALIAS_args
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS