Boost.cmake doesn't support late enough versions of MSVC. Separate ticket required, as it would be good to get very good coverage via travis and appveyor to ensure Python build is as portable as possible.
If we upgrade to Boost 1_68, then as of 1_66 we find that the 64/32 bit is encoded into the library file name, and then FindBoost on early versions fails to find it. Will probably leave this for now.
Reported by Johan.
After some investigation: