Hadrian: `-c/--configure` is broken again

30f6923a broke Hadrian -c:

Build system error - indirect recursion detected:
  Key value 1:  OracleQ (KeyValue ("hadrian/cfg/system.config","cross-comp
iling"))
  Key value 2:  hadrian/cfg/system.config
  Key value 3:  hadrian/cfg/system.config mk/config.h compiler/ghc.cabal rts/rts.cabal ghc/ghc-bin.cabal utils/iserv/iserv.cabal utils/iserv-proxy/iserv-proxy.cabal utils/remote-iserv/remote-iserv.cabal utils/runghc/runghc
.cabal utils/gen-dll/gen-dll.cabal libraries/ghc-boot/ghc-boot.cabal libraries/ghc-boot-th/ghc-boot-th.cabal libraries/ghci/ghci.cabal libraries/ghc-heap/ghc-heap.cabal libraries/libiserv/libiserv.cabal libraries/template-haskell/template-haskell.cabal docs/users_guide/ghc_config.py docs/index.html libraries/prologue.txt distrib/configure.ac
  Key value 4:  OracleQ (KeyValue ("hadrian/cfg/system.config","bourne-shell"))
Rules may not be recursive

bourne-shell value from hadrian/cfg/system.config is needed to run autoreconf, that is needed to build configure that is needed to build hadrian/cfg/system.config.