FindBoost: Restore tolerance of backslashes in paths

Since commit v3.8.0-rc1~136^2 (FindBoost: Search official location of
prebuilt binaries on Windows, 2016-12-21) we pass input paths through
`_Boost_UPDATE_WINDOWS_LIBRARY_SEARCH_DIRS_WITH_PREBUILT_PATHS` in more
places than before.  This broke tolerance of backslashes in paths
provided by the user due to the macro argument re-parsing.  Turn
`_Boost_UPDATE_WINDOWS_LIBRARY_SEARCH_DIRS_WITH_PREBUILT_PATHS` into a
function instead of macro to avoid re-parsing of macro arguments.

Fixes: #16816