We suspect (or at least I suspect) that this and the similar problem
with ruby-fftw3 are caused by a bug in fftw3 ("runtime detection of
NEON is perhaps broken"):
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=767138
This suggests a more general question that readers of this list might
be able to help us with: which other Debian packages do runtime
detection of NEON, and how do they do it?