o Minor bugfixes (portability):
    - On the chance that somebody manages to build Tor on a
      platform where time_t is unsigned, correct the way that
      microdesc_add_to_cache handles negative time arguments. Fix for
      bug 8042; bugfix on 0.2.3.1-alpha.

  o Build improvements:
    - Warn if building on a platform with an unsigned time_t: there
      are too many places where Tor currently assumes that time_t can
      hold negative values. We'd like to fix them all, but probably
      some will remain.