With deep gratitude to Ben and Moritz, who helped me on IRC, I’ve figured this out. Somehow my gmp install was broken -- a `brew install gmp` fixes it. What’s very bizarre is that the brokenness happened on two different computers, so it’s not terribly hard to do. Of course, both computers are maintained by me, so I may have done the same strange step twice.
Posting a ticket seems unlikely to be helpful, so I’ll park this issue for now, having solved it. But if anyone ever sees this again, the answer is `brew install gmp`.
Thanks,
Richard
No idea what this is, but just to point out that the continuous build succeeded on OS X at the same revision that you have:
https://phabricator.haskell.org/B16186, so this is at least not a universal failure. Something specific to the version of GMP, or some other external tool/library?