Always haddock by default

Revert this patch:
    Matthias Kilian <kili@outback.escape.de>**20090920181319
    Don't build haddock if HADDOC_DOCS = NO, and disable HADDOC_DOCS
        if GhcWithInterpreter = NO
    Haddock uses TcRnDriver.tcRnGetInfo, which is only available if
    GHCI is built. Set HADDOC_DOCS to NO if GhcWithInterpreter is NO,
    and disable the haddock build if HADDOC_DOCS = NO.