Cabal --builddir

Hi, Cabal documents itself as having a --builddir command to change the placement of dist stuff, which is fantastic. However, running with GHC 6.8.3, Cabal 1.6.0.2, I get: cabal build --builddir=../../../_make/Tools/ext/haddock-0.9/dist setup.exe: Unrecognised flags: --builddir=../../../_make/Tools/ext/haddock-0.9/dist i.e. cabal accepts the --builddir command, but the setup that cabal builds doesn't. Why? Thanks Neil