This is a pain now for extension writers on Win32, and will become crucial later, when 64 bit Windows becomes more popular. We *could* all just write a simple WIN32::API script within our Makefiles to get the major and minor version of Windows and set the value accordingly. The *better* option would be to have MakeMaker automatically set this value for me so I don't have to.
I haven't checked for certain, but I don't *think* Perl's ExtUtils::MakeMaker does this (nor does Ruby's mkmf). Time to submit a patch.