[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.

Proposed solution: explicitly qualify enable_if and disable_if with the boost namespace.

This problem has appeared in recent boost/geometry unit tests, on msvc-8.0 platforms, where the included version of the unit test is used.

For example see:
http://www.boost.org/development/tests/develop/developer/output/teeks99-02a-win2008-64on64-boost-bin-v2-libs-geometry-test-algorithms-difference_linear_linear-test-msvc-8-0-debug-asynch-exceptions-on-threading-multi.html
for the results of the unit test, and:
https://github.com/boostorg/geometry/blob/develop/test/algorithms/difference_linear_linear.cpp
for the source code of the unit test.