GN build improvements.

tools/gn/secondary/third_party/expat/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.
-
-if (is_linux) {
-  config("expat_config") {
-    libs = [ "expat" ]
-  }
-
-  group("expat") {
-    direct_dependent_configs = [ ":expat_config" ]
-  }
-
-  # TODO(brettw) Android needs direct dependent includes of
-  # <android_src>/external/expat/lib
-} else {
-  config("expat_config") {
-    include_dirs = [ "files/lib" ]
-    defines = [ "XML_STATIC" ]
-  }
-
-  static_library("expat") {
-    sources = [
-      "files/lib/expat.h",
-      "files/lib/xmlparse.c",
-      "files/lib/xmlrole.c",
-      "files/lib/xmltok.c",
-    ]
-
-    defines = [ "_LIB" ]
-    if (is_win) {
-      defines += [ "COMPILED_FROM_DSP" ]
-    } else {
-      defines += [ "HAVE_EXPAT_CONFIG_H" ]
-    }
-  }
-}