Add GN conversions for more GYP flags

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) {
   os_include = "linux"
-} else if (is_mac) {
+} else if (is_mac || is_ios) {
   os_include = "mac"
 } else if (is_win) {
   os_include = "win32"
 }
 
 config("libxml_config") {
   # Define LIBXML_STATIC as nothing to match how libxml.h (an internal header)
   # defines LIBXML_STATIC, otherwise we get the macro redefined warning from
   # GCC. ("defines" does "-DFOO" which defines the macro FOO as 1.)
   cflags = [ "-DLIBXML_STATIC=" ]
@@ skipping 158 matching lines @@
 
       # See http://crbug.com/138571#c8
       "-Wno-ignored-attributes",
     ]
   }
 
   include_dirs = [
     "$os_include",
   ]
 }