externals/boost: Update to 1.68.0

This updates the submodule to use 1.68.0. Notably, it gets rid of the
silly

"Info: Boost.Config is older than your compiler version - probably
nothing bad will happen - but you may wish to look for an update Boost
version. Define BOOST_CONFIG_SUPPRESS_OUTDATED_MESSAGE to suppress this
message."

message that spams the output of the build process on Windows.