Improvments to time-measurement and stats:

  - move all the platform-dependent timing related stuff into
    posix/GetTime.c and win32/GetTime.c, with the machine-indepent
    interface specified in GetTime.h.  This is now used by
    Stats.c.

  - On Unix, use gettimeofday() and getrusage() by default, falling
    back to time() if one of these isn't available.

  - try to implement thread-specfic CPU-time measurement using
    clock_gettime() on Unix.  Doesn't work reliably on Linux, because
    the implemenation tries to use the processor TSC, which on an
    SMP machine goes wrong when the thread moves between CPUs.  However,
    it's slightly less bogus that before, and hopefully will improve
    in the future.