Win32: Building with Longhorn SDK

When building with MSVC 2005 (Express) and the Longhorn Platform SDK
(version 6.0a), it seems that IPPROTO_IPV6 is only defined when
_WIN32_WINNT >= 0x0501. This results in a compiler error when trying to
build pqcomm.c (line 389). According to [1], building for Windows 2000
(that is, _WIN32_WINNT == 0x0500) is no longer supported with the
Longhorn version of the Platform SDK.

This isn't a huge problem (we can just require the use of prior versions
of the SDK), but I thought I'd mention it for the archives.

-Neil

[1]
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1960499&SiteID=1