Windows build problems

I'm trying to set up a build/test environment on Windows. Building ghc (sh validate) fails after a while due to flex and bison crashing. Those two come with git and even $ flex --version (or bison) crashes, so they seem truly hosed. Do I need flex/bison at all to build ghc? It seems they're not used on linux for building ghc. So, if not, how do I configure things that the build doesn't try to use bison/flex? If yes, would installing flex and bison from gnuwin32 work?