I created a new variable because, I think we don't want to loose wiredtiger 2.9.x which is used by mongodb 3.4.

wiredtiger 2.9 aka. wiredtiger variable is also used by guile-wiredtiger. I am going to update guile-wiredtiger

soon enough in another patch to use wiredtiger-3.

I don't know if mongodb 3.4 can be built with wiredtiger-3.1.0