Cleaning up the configuration/build process.

  - New build.mk option: GhcWithInterpreter. It has reasonable
    defaults, and shouldn't need to be overriden (you get the interpreter
    if you're bootstrapping with ghc 4.09+ on an ELF architecture, at
    the moment).

  - compilation manager now lives in compiler/compMan.  compiler/ghci
    contains only interpreter-related files.

  - WithGhcHc has gone, it now defaults to $(GHC).  This is so that
    we can reliably determine the version of $(GHC) using the stuff that
    configure tells us.  configure gets a new --with-ghc option so you
    can specify which ghc to use.