Various GN fixes.

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 156 matching lines @@
       # libxml passes `const unsigned char*` through `const char*`.
       "-Wno-pointer-sign",
 
       # pattern.c and uri.c both have an intentional `for (...);` /
       # `while(...);` loop. I submitted a patch to move the `'` to its own
       # line, but until that's landed suppress the warning:
       "-Wno-empty-body",
 
       # debugXML.c compares array 'arg' to NULL.
       "-Wno-tautological-pointer-compare",
+
+      # threads.c attempts to forward declare a pthread_equal which doesn't
+      # match the prototype in pthreads.h
+      "-Wno-ignored-attributes",
     ]
   }
 
   include_dirs = [
     "$os_include",
   ]
 }