this commit is working for all versions of boost supported:
* BOOST_NO_EXPLICIT_CONVERSION_OPERATORS is defined in boost 1.39->1.50
* BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS is defined startin boost 1.51
actually there is even a macro in boost setting
BOOST_NO_EXPLICIT_CONVERSION_OPERATORS when
BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS is defined so we could be fine
with only the first one.