[IOT-1561] Improve vs12_snprintf

[IOT-1561] Improve vs12_snprintf

Improve the snprintf surrogate used to build for Windows using Visual
Studio 2013 or older.

Also remove the dependency on cmake for building gtest for Windows, to
fix a build break for the new snprintf_test.cpp.

This is a cherry-pick of commit b2a398e from the 1.2-rel branch.

Change-Id: I70c9d0766a9a316a19cbd283671137e61ab88dbf
Signed-off-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14387
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14627