tor/changes/double-0-check

  o Build improvements (bizarre platform detection):
    - Try to detect it if we are ever building on a platform where
      memset(...,0,...) does not set the value of a double to 0.0.  Such
      platforms are permitted by the C standard, though in practice
      they're pretty rare (since IEEE 754 is nigh-ubiquitous). We don't
      currently support them, but it's better to detect them and fail
      than to perform erroneously.