Set LIBCXX_BUILDING_LIBCXXABI when buildng libc++.

third_party/libc++/BUILD.gn

 # 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.
 
 config("config") {
   include_dirs = [
     "trunk/include",
     "../libc++abi/trunk/include",
   ]
   cflags = [
@@ skipping 42 matching lines @@
     "//build/config/gcc:no_exceptions",
     "//build/config/gcc:symbol_visibility_hidden",
   ]
   configs += [
     ":config",
     "//build/config/compiler:no_chromium_code",
     "//build/config/compiler:rtti",
     "//build/config/sanitizers:sanitizer_options_link_helper",
   ]
 
+  defines = [ "LIBCXX_BUILDING_LIBCXXABI" ]
   ldflags = [ "-nodefaultlibs" ]
 
   # TODO(GYP): Remove "-pthread" from ldflags.
   # -nodefaultlibs turns -pthread into a no-op, causing an unused argument
   # warning. Explicitly link with -lpthread instead.
 
   libs = [
     "c",
     "gcc_s",
     "m",
@@ skipping 21 matching lines @@
     "-stdlib=libc++",
 
     # Normally the generator takes care of RPATH. Our case is special because
     # the generator is unaware of the libc++.so dependency. Note that setting
     # RPATH here is a potential security issue. See the following for another
     # example of this issue: https://code.google.com/p/gyp/issues/detail?id=315
     "-Wl,-R,\$ORIGIN/",
   ]
   lib_dirs = [ root_build_dir ]
 }