SERVER-48273 backported some changes from upstream yaml-cpp.

But didn't do it in a way recorded in git history of the affected files. It's just a script doing patches. We have the mongodb-forks repo to better track this sort of thing. I've replicated and committed the relevant cherry-picks in the mongodb-forks/yaml-cpp repo, under a mongo/0.6.2 branch. Further pulls from upstream can be merged with that branch.