Ensure that _GNU_SOURCE is defined early enough

sandbox/linux/suid/process_util_linux.c

 // Copyright (c) 2012 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.
 
 // The following is the C version of code from base/process_utils_linux.cc.
 // We shouldn't link against C++ code in a setuid binary.
 
+// Needed for O_DIRECTORY, must be defined before fcntl.h is included
+// (and it can be included earlier than the explicit #include below
+// in some versions of glibc).
+#define _GNU_SOURCE
+
 #include "sandbox/linux/suid/process_util.h"
 
-#define _GNU_SOURCE  // needed for O_DIRECTORY
-
 #include <fcntl.h>
 #include <inttypes.h>
 #include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ skipping 44 matching lines @@
   close(dirfd);
 
   char buf[11];  // 0 <= |score| <= kMaxOomScore; using log_10(2**32) + 1 size
   snprintf(buf, sizeof(buf), "%d", score);
   size_t len = strlen(buf);
 
   ssize_t bytes_written = write(fd, buf, len);
   close(fd);
   return (bytes_written == len);
 }