GN: refactor third_party/gni

third_party/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")
 
-# Most third_party code should disable warnings: if we don't maintain the code, warnings are noise.
-config("no_warnings") {
-  cflags = [ "-w" ]
-}
-third_party_configs = [ ":no_warnings" ]
-
 config("libpng_public") {
   include_dirs = [ "libpng" ]
 }
-source_set("libpng") {
+third_party("libpng") {
   public_configs = [ ":libpng_public" ]
-  configs += third_party_configs
-  configs -= unwanted_configs
 
   deps = [
-    ":zlib",
+    "//third_party/zlib",
   ]
   sources = [
     "libpng/png.c",
     "libpng/pngerror.c",
     "libpng/pngget.c",
     "libpng/pngmem.c",
     "libpng/pngpread.c",
     "libpng/pngread.c",
     "libpng/pngrio.c",
     "libpng/pngrtran.c",
     "libpng/pngrutil.c",
     "libpng/pngset.c",
     "libpng/pngtrans.c",
     "libpng/pngwio.c",
     "libpng/pngwrite.c",
     "libpng/pngwtran.c",
     "libpng/pngwutil.c",
   ]
 }
 
 config("libjpeg-turbo_config") {
   include_dirs = [ "externals/libjpeg-turbo" ]
 }
-source_set("libjpeg-turbo") {
+third_party("libjpeg-turbo") {
   public_configs = [ ":libjpeg-turbo_config" ]
-  configs += third_party_configs
-  configs -= unwanted_configs
 
   sources = [
     "externals/libjpeg-turbo/jcapimin.c",
     "externals/libjpeg-turbo/jcapistd.c",
     "externals/libjpeg-turbo/jccoefct.c",
     "externals/libjpeg-turbo/jccolor.c",
     "externals/libjpeg-turbo/jcdctmgr.c",
     "externals/libjpeg-turbo/jchuff.c",
     "externals/libjpeg-turbo/jcinit.c",
     "externals/libjpeg-turbo/jcmainct.c",
@@ skipping 42 matching lines @@
 group("freetype2") {
   public_configs = [ ":system_freetype2" ]
 }
 
 config("system_fontconfig") {
   libs = [ "fontconfig" ]
 }
 group("fontconfig") {
   public_configs = [ ":system_fontconfig" ]
 }