Fix Glib flag in GN build.

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 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.
 
 import("//build/config/android/config.gni")
 import("//build/config/sysroot.gni")
 
 # compiler ---------------------------------------------------------------------
 #
 # Base compiler configuration.
@@ skipping 113 matching lines @@
       "-pipe",  # Use pipes for communicating between sub-processes. Faster.
     ]
     if (!is_android) {
       cflags += [ "-pthread" ]
     }
 
     if (cpu_arch == "x64") {
       # Enable libstdc++ debugging facilities to help catch problems early, see
       # http://crbug.com/65151 .
       # TODO(phajdan.jr): Should we enable this for all of POSIX?
-      defines += [ "__GLIBCXX_DEBUG=1" ]
+      defines += [ "_GLIBCXX_DEBUG=1" ]
 
       # Use gold for linking on 64-bit Linux only (on 32-bit it runs out of
       # address space, and it doesn't support cross-compiling).
       gold_path = rebase_path("//third_party/gold", ".", root_build_dir)
       ldflags += [
         "-B$gold_path",
 
         # There seems to be a conflict of --icf and -pie in gold which can
         # generate crashy binaries. As a security measure, -pie takes
         # precendence for now.
@@ skipping 472 matching lines @@
   } else {
     cflags = [ "-g1" ]
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }