[Haskell-cafe] Trouble building happy/alex with Stack lts-9.20

Hi folks, I'm trying to update my language project to happy >= 9.16, so I can use typeclass-based parsers. Unfortunately, I'm running into build errors after switching to lts-9.20. The log file is attached. I see similar problems when trying to build alex (the lexer generator). It seems to be claiming that there is no cabal file, which is odd, as there clearly is one (in the source at least). Has anyone else seen this problem, and if so, what is the workaround?