GHC extensions

Friends As part of the reinstallable base project I'm adding an extension "ImplicitKnownKeyNames" to GHC. But if I put "NoImplicitKnownKeyNames" in base.cabal.in I get the error Warning: [unknown-extension] Unknown extensions: NoImplicitKnownKeyNames when building the base library. I think this comes from cabal. How can I tell cabal about the new extension? Thanks Simon