Add BOOST_LIBSTDCXX_VERSION_WORKAROUND_GUARD

[pdimov]

This avoids the -Wundef warning when BOOST_WORKAROUND(BOOST_LIBSTDCXX_VERSION, ...) is used without a defined(BOOST_LIBSTDCXX_VERSION) check.

[pdimov]

Merging because the -Wundef warning is unavoidable on the user side even if guarded.

./boost/unordered/detail/type_traits.hpp:66:58: error: "BOOST_LIBSTDCXX_VERSION_WORKAROUND_GUARD" is not defined [-Werror=undef]
 #if defined(BOOST_LIBSTDCXX_VERSION) && BOOST_WORKAROUND(BOOST_LIBSTDCXX_VERSION, < 50000)
                                                          ^