[krbdev.mit.edu #5453] Windows - some apps define ssize_t as a preprocessor symbol

[Jeffrey Altman via RT]

revision 19123 added a typedef declaration for ssize_t which
is not declared by Microsoft in versions of visual studio
and the associated C Run Time Libraries.  Following the style
used by Microsoft new type declarations are wrapped with a
pre-processor macro so it is possible to determine if the
typedef is present.

Not all third party applications and libraries are careful to
follow the pattern set by Microsoft.  As a result, the inclusion
of ssize_t in win-mac.h was broken by libraries that include
krb5.h after defining ssize_t as a preprocessor macro.  This
patch undefines the macro prior to defining ssize_t as a new
type.