o Build fixes (MSVC):
    - Correctly define HAVE_EVENT_BASE_LOOPEXIT, since we only build
      with MSVC when using Libevent 2.0 or later. Fixes bug 7308.
      Reported by "ultramage".
    - Make the ntor and curve25519 code build correctly with MSVC.
      Fix on 0.2.4.8-alpha.