I noticed by perusing
http://buildfarm.postgresql.org/cgi-bin/show_members.pl
that several animals are up-to-date on building the back branches,
but haven't built HEAD in some time:
anole (72 days back)
castoroides (74 days back)
hyrax (165 days back)
mule (275 days back)
narwhal (48 days back)
Please check that, if any of these belong to you. These critters are not
dead, because they've all reported on one or more back branches recently,
but they must be misconfigured somehow.
regards, tom lane