Unfortunately, it looks like my cabal build failure occurs in a temporary and very short-lived directory. So presumably the dodgy FFI gets copied into there from elsewhere. I wonder if I can find the source...
It's running an executable it seems to have built to generate something else for the build, so I suspect you are in fact seeing the ghc bug and you should fix the ghc reference. If you installed the official HP package, you need to find libHSrts-ghc7.4.2.dylib somewhere under /Library/Haskell and use install_name_tool on that.