configure: be explicit about "XP" Windows API version

configure: be explicit about "XP" Windows API version

With a Cygwin Mingw-w64 environment, when an explicit Windows API
version define isn't set the Windows Server 2003 API is targeted
(https://github.com/mirror/mingw-w64/blob/a25dc933f3d3ed6c8529fb24b7ae393f8792ca69/mingw-w64-headers/include/sdkddkver.h#L166
). This means the XP build was actually misnamed and have never been
fully compatible with plain XP for many years (I suspect such binaries
will likely still work on XP bar trying to use the net ioengine)...

With an MSYS2 Mingw-w64 environment, the default Windows API version is
Windows 7
(https://github.com/msys2/MINGW-packages/blob/e9f271c61a39076ab5de56299f7fd86020c280ae/mingw-w64-headers-git/PKGBUILD#L55
) which breaks an "XP" build because we were were relying on the default
being 2003. Fix this by making the "XP" build explicitly target the
Windows Server 2003 API.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>