Add librt to default libs on linux

base/third_party/libevent/BUILD.gn

 # 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.
 
 import("//build/config/nacl/config.gni")
 
 source_set("libevent") {
   sources = [
     "buffer.c",
     "evbuffer.c",
@@ skipping 11 matching lines @@
   ]
 
   defines = [ "HAVE_CONFIG_H" ]
 
   if (is_mac || is_ios) {
     sources += [ "kqueue.c" ]
     include_dirs = [ "mac" ]
   } else if (is_linux) {
     sources += [ "epoll.c" ]
     include_dirs = [ "linux" ]
-    libs = [ "rt" ]
   } else if (is_android) {
     sources += [ "epoll.c" ]
     include_dirs = [ "android" ]
   } else if (is_nacl_nonsfi) {
     sources -= [
       "evdns.c",
       "event_tagging.c",
       "evrpc.c",
       "http.c",
       "select.c",
       "signal.c",
     ]
     sources += [
       "nacl_nonsfi/config.h",
       "nacl_nonsfi/event-config.h",
       "nacl_nonsfi/random.c",
       "nacl_nonsfi/signal_stub.c",
     ]
     include_dirs = [ "nacl_nonsfi" ]
   }
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }