GN: refactor third_party/gni

third_party/zlib/BUILD.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+declare_args() {
+}
+
 import("../third_party.gni")
 
 config("zlib_public") {
   include_dirs = [ "../externals/zlib" ]
 }
-source_set("zlib") {
+third_party("zlib") {
   public_configs = [ ":zlib_public" ]
-  configs -= unwanted_configs
-
-  cflags = [ "-w" ]
 
   sources = [
     "../externals/zlib/adler32.c",
     "../externals/zlib/compress.c",
     "../externals/zlib/crc32.c",
     "../externals/zlib/deflate.c",
     "../externals/zlib/gzclose.c",
     "../externals/zlib/gzlib.c",
     "../externals/zlib/gzread.c",
     "../externals/zlib/gzwrite.c",
     "../externals/zlib/infback.c",
     "../externals/zlib/inffast.c",
     "../externals/zlib/inflate.c",
     "../externals/zlib/inftrees.c",
     "../externals/zlib/simd_stub.c",
     "../externals/zlib/trees.c",
     "../externals/zlib/uncompr.c",
     "../externals/zlib/zutil.c",
   ]
 }