Hadrian build fails on FreeBSD

A plain hadrian build using hadrian/build.cabal.sh bootstrapped with GHC 8.4.3 from ports fails with

/---------------------------------------------------------------------------------\
| Successfully built program 'ghc-pkg' (Stage0).                                  |
| Executable: _build/stage0/bin/ghc-pkg                                           |
| Program synopsis: A utility for querying and managing the GHC package database. |
\---------------------------------------------------------------------------------/
hadrian: '/usr/home/ben/ghc/_build/stage0/bin/ghc' exited with an error:
ghc: missing -B<dir> option

shakeArgsWith     0.000s    0%                           
Function shake    0.392s    0%                           
Database read     0.000s    0%                           
With database     0.000s    0%                           
Running rules  1725.240s   99%  =========================
Total          1725.633s  100%                           
Error when running Shake build system:
  at src/Rules.hs:(32,19)-(45,17):
  at src/Rules.hs:45:5-17:
* Depends on: _build/stage1/lib/bin/unlit
  at src/Development/Shake/Internal/Rules/Oracle.hs:157:43-68:
* Depends on: OracleQ (PackageDataFile (Context {stage = Stage1, package = Package {pkgLanguage = Haskell, pkgType = Program, pkgName = "unlit", pkgPath = "utils/unlit"}, way = v}))
  at src/Hadrian/Haskell/Cabal/Parse.hs:200:5-36:
* Depends on: _build/stage1/utils/unlit/setup-config
* Raised the exception:
ExitFailure 1

on FreeBSD 11 on amd64.