tor/changes/signof_enum

  o Code simplifications and refactoring:
    - Use Ville Laurikari's implementation of AX_CHECK_SIGN() to determine
      the signs of types during autoconf. This is better than our old
      approach, which didn't work when cross-compiling.
    - Detect the sign of enum values, rather than assuming that MSC is the
      only compiler where enum types are all signed. Fix for bug 7727;
      bugfix on 0.2.4.10-alpha.