RFC: Hide zlib and expat libs from the user of Poco libraries

[teksturi]

Seperete zlib and expat so they are only expose internally. There should not be reason for user to use these.

This makes https://github.com/pocoproject/poco/issues/4358 easier to implement as all external libraries are not exposed to end user.

[teksturi]

Please say if we example still want to expose ParserEngine. I will try to make necessary changes for that. I do not remember exactly if that would be easy as this has been quite long in my POC box.

Also if this is path we will take I also need to check VS project files more closely.

[matejk]

@teksturi , any updates on this?

[matejk]

Replaced with #4724.