Set LIBCXX_BUILDING_LIBCXXABI when buildng libc++.

third_party/libc++/libc++.gyp

 # Copyright 2015 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.
 
 {
   'targets': [
     {
       'target_name': 'libcxx_proxy',
       'type': 'none',
       'toolsets': ['host', 'target'],
@@ skipping 84 matching lines @@
         '-pthread',
         '-std=c++11',
       ],
       'cflags_cc!': [
         '-fno-exceptions',
         '-fno-rtti',
       ],
       'cflags!': [
         '-fvisibility=hidden',
       ],
+      'defines': [
+        'LIBCXX_BUILDING_LIBCXXABI',
+      ],
       'ldflags': [
         '-nodefaultlibs',
       ],
       'ldflags!': [
         # -nodefaultlibs turns -pthread into a no-op, causing an unused argument
         # warning. Explicitly link with -lpthread instead.
         '-pthread',
       ],
       'libraries': [
         '-lc',
         '-lgcc_s',
         '-lm',
         '-lpthread',
         '-lrt',
       ],
     },
   ]
 }