title: issue with boost json parser
body: <p>I am trying to use boost <code>json_parser</code>. I have included <code>boost/property_tree/json_parser.hpp</code> in my <code>cpp</code> file.</p>
  
  <p>I have not included any other code. But I am getting many compilation  errors  like error: </p>
  
  <blockquote>
  <pre><code>boost::spirit::classic' has not been declared and
      In function void boost::property_tree::json_parser::read_json_internal(std::basic_istream&lt;typename
  Ptree::key_type::value_type, std::char_traits&lt;typename
  Ptree::key_type::value_type&gt; &gt;&amp;, Ptree&amp;, const std::string&amp;):
       [exec] /usr/include/boost/property_tree/detail/json_parser_read.hpp:288:
  error: 'classic' is not a namespace-name
  </code></pre>
  </blockquote>
  
  <p>Can you help me resolve this issue?</p>