[boost] fix link issue with boost_system when statically linked

[matlo607]

On Windows, symbols related to boost::system are missing when using a static version of boost-system.
BOOST_ERROR_CODE_HEADER_ONLY must be defined because of this piece of code in boost/system/error_code.hpp.

#ifdef BOOST_ERROR_CODE_HEADER_ONLY
    inline const error_category &  system_category() BOOST_SYSTEM_NOEXCEPT;
    inline const error_category &  generic_category() BOOST_SYSTEM_NOEXCEPT;
#else
    BOOST_SYSTEM_DECL const error_category &  system_category() BOOST_SYSTEM_NOEXCEPT;
    BOOST_SYSTEM_DECL const error_category &  generic_category() BOOST_SYSTEM_NOEXCEPT;
#endif

If BOOST_ERROR_CODE_HEADER_ONLY is not defined, symbols of system_category() and generic_category() are prepended with __imp_ (due to BOOST_SYSTEM_DECL -> __declspec(dllimport)) and not found when linking against libcucumber-cpp.lib

[coveralls]

Coverage remained the same at 62.814% when pulling 6db8b21 on matlo607:fix-link-boost-system into 79e92de on cucumber:master.

[matlo607]

@muggenhor, @konserw : please could somebody review this PR ?

[matlo607]

Please could somebody review this PR ?
Is there anything blocking ?