vchoi_fork

[svn-r4796] Purpose:
    Bug fix
Description:
    The value of H5_SIZEOF_SSIZE_T is not being updated to reflect the correct
    size of the ssize_t typedef, if we have to define it ourselves.
Solution:
    Undef H5_SIZEOF_SSIZE_T at beginning of block where we define the typedef
    for ssize_t and then re-define it to the correct size when we've chosen a
    size.
Platforms tested:
    FreeBSD 4.5 (sleipnir)