Add build target for nacl_io from the NaCl SDK.

native_client_sdk/src/libraries/nacl_io/syscalls/fdopen.c

 /* 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. */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <stdio.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 #if defined(__native_client__) && defined(__GLIBC__)
 
 static ssize_t fdopen_read(void *cookie, char *buf, size_t size) {
   return read((int) cookie, buf, size);
 }
 
@@ skipping 25 matching lines @@
  * TODO(bradnelson): Remove this glibc is fixed.
  * See: https://code.google.com/p/nativeclient/issues/detail?id=3362
  * Workaround fdopen in glibc failing due to it vetting the provided file
  * handles with fcntl which is not currently interceptable.
  */
 FILE *fdopen(int fildes, const char *mode) {
   return fopencookie((void *) fildes, mode, fdopen_functions);
 }
 
 #endif  /* defined(__native_client__) && defined(__GLIBC__) */