(Re)define ssize_t as make_signed<size_t>

On macOS, the previous code chose int64_t as ssize_t, and int64_t is
defined as long long, but ssize_t had been defined as long (despite also
being 8 bytes).  This caused the typedef to be non-idempotent.

make_signed<size_t> chooses the right counterpart, depending on the base
type of size_t, e.g. unsigned long -> long, unsigned long long -> long
long, which is more robust when there are distinct base integer types
with the same size.