[xml] static build of libxml2 on windows

[<oliverst online de>]

I wanted to change the configure script for windows, so it also builds
the static lib with the "LIBXML_STATIC_FOR_DLL" define, what is quite
essential to get DLL with a static libxml2 in it to work properly.
Now I just wonder what would be the better way.
Add a third state of static (add "dll" to existing "no" and "yes") or
just build an additional lib named "libxml2_a_dll.lib" (for MSVC) if
static is set to "yes"?