WIP: make guile bindings compile with guile 2.2.2

Open username-removed-595144 requested to merge rowanthorpe/gnutls:latest-guile into master

As per the title, this is a WIP merge request (to prevent potential duplication of effort) - WIP because I am not entirely sure I'm doing the right thing, and while I'm soliciting help by email for the last part. The associated "issue" for this is #199.