Updates for rtools40

[jeroen]

This prepares the script for rtools40 in R >= 4.0. I write it as a separate function because rtools40 is a bit different from previous versions.

Perhaps we can tweak it more later, but this should fix the immediate builds.

[krlmlr]

Thanks! This breaks builds on R < 3.0.0, we'll find out if that matters.

[jeroen]

Thanks!