Odd uuid-ossp behavior on smew and shearwater

Buildfarm critters smew and shearwater are reporting regression test
failures that suggest that the UUID library can't get a system MAC
address:
http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=smew&dt=2014-05-28%2023%3A38%3A28
http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=shearwater&dt=2014-05-29%2000%3A24%3A32

I've just committed regression test adjustments to prevent that from
being a failure case, but I am confused about why it's happening.
I wouldn't be surprised at not getting a MAC address on a machine that
lacks any internet connection, but that surely can't describe the
buildfarm environment.  Are you curious enough to poke into it and
see what's going on?  It might be useful to strace a backend that's
trying to execute uuid_generate_v1() and see what the kernel interaction
looks like exactly.
        regards, tom lane