GN build improvements.

tools/gn/secondary/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.
-
-static_library("libevent") {
-  sources = [
-    "buffer.c",
-    "epoll.c",
-    "epoll_sub.c",
-    "evbuffer.c",
-    "evdns.c",
-    "event.c",
-    "event_tagging.c",
-    "evrpc.c",
-    "evutil.c",
-    "http.c",
-    "kqueue.c",
-    "log.c",
-    "poll.c",
-    "select.c",
-    "signal.c",
-    "strlcpy.c",
-  ]
-
-  defines = [ "HAVE_CONFIG_H" ]
-
-  if (is_linux) {
-    libs = [ "rt" ]
-  } else {
-    sources -= [ "epoll.c", "epoll_sub.c" ]
-  }
-
-  if (is_mac) {
-    include_dirs = [ "mac" ]
-  } else if (is_linux) {
-    include_dirs = [ "linux" ]
-    sources -= [ "kqueue.c" ]
-  } else if (is_android) {
-    include_dirs = [ "android" ]
-    sources -= [ "kqueue.c" ]
-  }
-
-  configs -= [ "//build/config/compiler:chromium_code" ]
-  configs += [ "//build/config/compiler:no_chromium_code" ]
-}