Revert 266932 "GN build improvements."

trunk/src/tools/gn/secondary/third_party/libxml/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.
 
 # Define an "os_include" variable that points at the OS-specific generated
 # headers.  These were generated by running the configure script offline.
 if (is_linux || is_android) {
   os_include = "linux"
 } else if (is_mac || is_ios) {
   os_include = "mac"
@@ skipping 170 matching lines @@
         # debugXML.c compares array 'arg' to NULL.
         "-Wno-tautological-pointer-compare",
       ]
     }
   }
 
   include_dirs = [
     "$os_include",
   ]
 }