Bug#783920: atlas: FTBFS on arm64 in Jessie

[Edmund Grimley Evans]

Firstly, I wasn't able, just now, to reproduce the build failure in
either jessie or unstable.

Secondly, I found a couple of old build logs from March. They were
done with sbuild on different but identical systems, and there is no
difference in the versions of the packages installed as reported near
the start of the logs. However, one build succeeded, and the other
failed in the manner described above. So it looks as though the
problem is a race condition or something similarly difficult to
reproduce.

I'm not sure whether this is directly relevant, but one of the logs
has "/usr/bin/c99-gcc" where the other has "/usr/bin/gcc-4.9", in
several thousand places. I don't particularly suspect that that's the
cause of the failure but it's another sign of the build being
non-deterministic.