And the answer is ... it's a gmake bug. Apparently introduced in 3.82.
http://savannah.gnu.org/bugs/?30653
https://bugzilla.redhat.com/show_bug.cgi?id=835424
So I think .NOTPARALLEL is just masking the true problem, but
nonetheless it's a problem. And given that the bug report on savannah
has been ignored for two years, we should not hold our breath for a fix
to appear upstream (much less propagate to everyone using 3.82).
regards, tom lane