Improve the BOOST_FUSION_ADAPT_STRUCT macro to work also when there is no variadic support.

This enable the following flexible macro signature when there is
no BOOST_PP_VARIADICS support:

BOOST_FUSION_ADAPT_STRUCT(
    struct_name,
    (member_type0, member_name0)
    (BOOST_FUSION_ADAPT_AUTO, member_name1)
    (,member_name2)
    ...
    )

If BOOST_PP_VARIADICS is active then there is quite more flexibility as it
allows to avoid the type specification:

BOOST_FUSION_ADAPT_STRUCT(
    struct_name,
    (member_type0, member_name0)
    (BOOST_FUSION_ADAPT_AUTO, member_name1)
    (,member_name2)
    (member_name3)
    ...
    )