Don't link combase.dll on win7 target

[roblabla]

Since #3743 was merged, win7 builds have been broken due to linking functions from the combase.dll which doesn't exist on win7 (the functions are instead in ole32.dll).

This PR fixes the issue by checking for target_vendor to chose whether to link against ole32 or combase.

[kennykerr]

Thanks for the suggestion, but Windows 7 is not supported by this project. It looks like @riverar offered a workaround in #3743 that may be worth exploring.

[roblabla]

This project is absolutely foundational to the Rust ecosystem. I understand not wanting to advertise or provide win7 support yourself, but why refuse to pull a simple patch that fixes the immediate defect with no impact to the normal rust targets? Especially for a crate whose only purpose is providing language bindings, this seems to have minimal maintenance impact.

I saw the suggestion after I did the change, but this requires every app that wants a win7 build to pull this patch, greatly reducing the usefulness of the Rust win7 target.

I hope you will reconsider this position.