ssize_t for Visual Studio

The POSIX C type “ssize_t” is available on Unix-like systems in unistd.h. For Windows Visual Studio based compilers without unistd.h, the following stanza allows use of ssize_t.

#ifdef _MSC_VER
#include <BaseTsd.h>
typedef SSIZE_T ssize_t;
#else
#include <unistd.h>
#endif