Visual Studio 14 CTP defines snprintf()

For https://bugzilla.gnome.org/show_bug.cgi?id=737937

Visual Studio 14 CTP (the VS which comes with Windows 10) defines snprintf().
It could be seen as a good idea as snprintf() is part of the C99 standard but
unfortunately libxml2 as many packages defines snprintf as _snprintf, the
function to use for any previous versions of the Visual Studio runtime. More,
to avoid hiding/shadowing snprintf() declaration in stdio.h is protected by an
"#ifdef snprintf" followed by an "#error", so compilation fails.

But the fix is easy: the corresponding C/C++ compiler defines _MSC_VER to 1900
so it is enough to guard the snprintf define against it, cf. the attached patch
for win32config.h (from 2.9.1 "latest" tarball).