Ask HN: How to made a Windows-tied library cross-platform?

Our main language (Delphi) will be, most probably, cross-platform in its next version.

While writing a cross-compiler isn't a thing which interests us very much now, porting a library which was/is Windows-tied to multiple platforms, /certainly/ does.

You can think, for example at VCL (Delphi's standard library). While it was designed for Windows only, it has value in it, and, of course, there are huge codebases which depend on it.

The question is: Which would be the best approach to made an application / library cross-platform aware ensuring a smooth conversion / upgrade path (as much as possible of course)?

I stress it again, we are not interested which is the best way to do cross-platform development only (there were questions on this theme). We are interested also in yet another requirement: The old code base / installations management.

PS: Experiences and/or methodologies from similar situations with other languages (eg. C/C++) which are regarded as standard practices are welcomed.

Thanks in advance.