Some win32 compilers define the SSIZE_T type, with the same
meaning and intent as ssize_t. If available, make ssize_t a
synonym of SSIZE_T.

At present, the Digital-Mars compiler is known not to define
SSIZE_T, so we provide an SSIZE_T macro to use in the typedef.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>