Several days ago, I fetched "network-manager-l2tp" and "openfortivpn"source from Debian testing/sid and tried to build them for Stretch,using sbuild. Basically, what I did was1, enable "testing" or "sid" in sources.list;2, "apt update";3, fetch the source of the package.4, sbuild the package with "-d stretch"I was able to generated guile_2.2.3-3 package with above steps. Butfor "network-manager-l2tp" and "openfortivpn", if I build directlyfrom the unchanged source from Sid, a complaint saying something like"dephelper (>= 11) was needed" was in the build log.I tried sbuild with ' --extra-repository="debhttp://httpredir.debian.org/debian stretch-backports main" '.dephelper (>= 11) was not met.I tried manually adding stretch-backports in sources.list in theschroot and then sbuild. dephelper (>= 11) was not met.
This is because the Backports Release file has "NotAutomatic: yes", which means that apt won’t get package dependencies from there. However, aptitude will. So you could call sbuild like this: