How to get user passed options from a Requirement?

As for `build.with?`, the thing is that boost build both single and multi-threaded flavors by default, so an option is provided to disable the single thread flavor.

Speaking of which, Debian now using system layout to only provide multi-threaded build without "-mt" suffix. I used to against this, but it seems to work fine in Debian, so I kind of changed my mind. Will discuss this in a separate PR.