Re-allow building on Microsoft Visual Studio 2013.

Re-allow building on Microsoft Visual Studio 2013.

In commit 450ee7012 I supposed that all platforms we now care about have
snprintf(), since that's required by C99.  Turns out that Microsoft did
not get around to adding that until VS2015.  We've dropped support for
VS2013 as of HEAD (cf 6203583b7), but not in the back branches, so add
a hack for this in the back branches only.

There's no easy shortcut to an exact emulation of standard snprintf
in VS2013, but fortunately we don't need one: this code was just fine
with using sprintf before 450ee7012, so we can make it do so again
on that platform (and any others where the problem might crop up).

Per bug #17681 from Daisuke Higuchi.  Back-patch to v12, like the
previous patch.

Discussion: https://postgr.es/m/17681-485ba2ec13e7f392@postgresql.org