unbundle: fix build with system zlib

build/linux/unbundle/zlib.gn

 # Copyright 2016 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/shim_headers.gni")
 
 shim_headers("zlib_shim") {
   root_path = "."
   headers = [ "zlib.h" ]
 }
 
+config("system_zlib") {
+  defines = [ "USE_SYSTEM_ZLIB=1" ]
+}
+
 source_set("zlib") {
   deps = [
     ":zlib_shim",
   ]
   libs = [ "z" ]
+  public_configs = [ ":system_zlib" ]
 }
 
 shim_headers("minizip_shim") {
   root_path = "contrib"
   headers = [
     "minizip/crypt.h",
     "minizip/ioapi.h",
     "minizip/iowin32.h",
     "minizip/mztools.h",
     "minizip/unzip.h",
@@ skipping 25 matching lines @@
 
 static_library("compression_utils") {
   sources = [
     "google/compression_utils.cc",
     "google/compression_utils.h",
   ]
   deps = [
     ":zlib",
   ]
 }