Don't add exe extension to builder paths on Windows.
Don't add exe extension to builder paths on Windows.

See #221, #222.

izgzhen/hadrian,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian,sdiehl/ghc,sdiehl/ghc,bgamari/shaking-up-ghc,sdiehl/ghc