Use the proper snprintf function if compiling with VS2015.

Use the proper snprintf function if compiling with VS2015.

Proper snprintf is finally supported as of the latest Visual Studio,
and has better semantics for my purposes than the old MS-specific
_snprintf. (Specifically, if its output doesn't fit the buffer, it
returns the full size it _would_ have wanted, so that you can then
immediately allocate that much space, and don't have to keep going
round a loop increasing the buffer size until you find the answer.)