Add support for boost versions without 'BOOST_NOEXCEPT'.

BOOST_NOEXCEPT is not in boost 1.47.0 in particular, which pion still
supports. Defining 'BOOST_SYSTEM_NOEXCEPT' to nothing in this case
will cause no harm.
Merge pull request #51 from camio/support_old_boost

Add support for boost versions without 'BOOST_NOEXCEPT'.