Ensure that _GNU_SOURCE is defined early enough

sandbox/linux/suid/linux_util.c

 // Copyright (c) 2009 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 duplicated from base/linux_utils.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/linux_util.h"
 
-#define _GNU_SOURCE  // For O_DIRECTORY
-
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.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 106 matching lines @@
         }
       }
     }
     closedir(fd);
     close(proc_pid_fd);
   }
   closedir(proc);
 
   return already_found;
 }