ext/Win32/Win32.xs compilation fails on msvc < 6

How about the attached patch?

This incorporates the #defines apparently required for MSVC
< 6, and
tests each one individually and in numerical order. To me it
makes more
sense than setting whole groups of #defines based on the
#definedness of
CSIDL_COMMON_FAVORITES and CSIDL_ADMINTOOLS, and it makes it
easier to
see what's there and what isn't.