Great news! Yesterday I tried to compile a C extension on Windows for Ruby and figured out the following problem myself (as described here):

The problem with this flexibility is that programs compiled and linked against version 6.0 cannot be called or used in programs linked with version 7.1 or 8.0, and vice versa.

So, if you’re not the owner of an old VC++ 6.0 compiler you will not succeed (the C++ compiler is free, but you just can’t find a download for it anymore).