Avoid CMake name clash for Boost.Thread library

FindBoost uses `Boost_THREAD_LIBRARY` to denote the library filename to
link when using Boost.Thread.

HPX_SetupBoost.cmake used `BOOST_THREAD_LIBRARY` as the target to detect
the underlying platform threading library used by Boost.Thread. As
find_library avoids the search if the cache variable exists, this search
was always a no-op.

The temporary name is now instead `BOOST_UNDERLYING_THREAD_LIBRARY`.