Make the workaround in snprintf.h conditional on older MSVC version.

Investigation reveals that the GetShortPathNameW is presumably used by
legacy Duden extension that was built by Sun in a setsolar environment.
The uwinapi.lib was never part of the SDK, so ordinary extensions cannot
link against it. The PRIVATE export of the symbol that is only possible
with DEF file therefore does not matter much. With VC2015 there is no
need for LO code to use uwinapi.dll at all and gb_UWINAPI can be unset
in this case.

Change-Id: I23a5636db9431b0fa4350b665a6a69613d625264
Reviewed-on: https://gerrit.libreoffice.org/16677Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>