Make XML compilation conditional.

gyp/xml.gyp

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'targets': [
     {
       'target_name': 'xml',
       'product_name': 'skia_xml',
       'type': 'static_library',
@@ skipping 17 matching lines @@
         '<(skia_src_path)/xml/SkXMLWriter.cpp',
       ],
       'sources!': [
           # time to kill this?
           '<(skia_src_path)/xml/SkXMLPullParser.cpp', #if 0 around class decl in header
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/xml',
         ],
+        'defines' : [
+          'SK_XML',
+        ],
       },
     },
   ],
 }