Upstream tarballs can now be used without a laundry list of patches, so build from an upstream tarball. (They are not yet publishing their tarballs though, so for now it still lives in releng.)
(This is marked WIP because GNOME/gjs!199 (merged) is still pending. Review is nonetheless welcome, as I am not sure how to test this other than merging it. Note that the download URL won't exist until someone runs signal-ftp-sync on the FTP server, as I have uploaded that tarball but I don't think I have permission to sync.)