[typeof][msvc-8.0] bug fix related to problem with included version of Boost.Test on msvc-8.0;
when both boost/utility/enable_if.hpp and boost/test/included/unit_test.hpp are included,
msvc-8.0 cannot figure out whether the enable_if and disable_if used in te typeof implementation
for msvc-8.0 is that in the boost namespace or that in the boost::unit_test::decorator namespace;
solution: explicitly qualify enable_if and disable_if with the boost namespace.