ECMGenerateExportHeader: add generation of *_DEPRECATED_VERSION_BELATED()

Summary:
Now and then tagging some API as deprecated for the compiler is forgotten.
Doing this retractivitly in newer versions but using the official version
might break build setups configured to only show warnings up to a certain
version and otherwise fail a build, using -Werror=deprecated-declarations.

To allow retroactive tagging of API for compiler warnings, and showing the
official version in the warniung message, while reacting only to warning
controls for the current version where the tag is added, avoids any such
annoying experiences, without wrong version info at the same time.

Reviewers: Frameworks, Build System, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D29573