[json_parser] preprocessor error if boost 1.49

* https://svn.boost.org/trac/boost/ticket/6785
* http://en.cppreference.com/w/cpp/preprocessor/error
* http://www.boost.org/doc/libs/1_54_0/libs/config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.boost_informational_macros