Useless dependency assumption libxml2 -> libxslt in MSVC scripts

Hi all,

Looking at the MSVC scripts for some stuff I have noticed the following thing:
    if ($options->{xml})
    {
        if (!($options->{xslt} && $options->{iconv}))
        {
            die "XML requires both XSLT and ICONV\n";
        }
    }
But I don't understand the reason behind such a restriction to be
honest because libxml2 does not depend on libxslt. The contrary is
true: libxslt needs libxml2. Note as well that libxml2 does depend on
ICONV though. So I think that this condition should be relaxed as
follows:
    if ($options->{xml} && !$options->{iconv})
    {
        die "XML requires ICONV\n";
    }
And we also need to be sure that when libxslt is specified, libxml2 is
here to have the build working correctly.

Relaxing that would allow people to compile contrib/xml2 with just a
dependency to libxml2, without libxslt, something possible on any *nix
systems. As far as I can see this restriction comes from 9 years ago
in 2007 and commit 7f58ed1a. So nobody has complained about that so
far :)

Attached is a patch to address both issues.
Comments are welcome.
--
Michael