Setting up a sandboxed installation of GHC HEAD

I'm trying to set up a sandboxed installation of GHC HEAD with cabal-install and some other libraries. I need this to run some benchmarks on Repa library. Right now I'm having problems with bootstraping cabal-install. I managed to work around problems with some pakcages like HTTP that put upper bound on base version but I'm stuck with cabal-install itself. I modified dependency on unix package but now I'm getting some compilation errors. I could probably fix them but I guess that this is not the correct way of setting up a working GHC HEAD installation. So the question is how should I install cabal-install with HEAD ? Janek