Semantically this is an ECM module, because it can be used to manage deprecated
Qt API and from any project which use ecm_generate_export_headers

This will make setting the deprecation versions easier, otherwise one would need
to edit the hex value. It also helps to keep the required versions and deprecation versions
in sync.

Also this allows one to keep the deprecation warnings, when one excludes deprecations for a specific version.

Additionally the deprecation version can be overwritten by a cmake parameter.
This will make local testing easier, because one does not need to edit the CMakeLists.txt files.

Task: https://phabricator.kde.org/T15109

This factors out large parts of the common code into separate modules,
and adds a backward compatibility wrapper.

The 6 variant drops some deprecated variables where possible, but otherwise
is the same as the 5 variant. It still lacks a replacement for the paths
depending on ECMQueryQMake though.

Thanks to kossebau for pointing that out.

NO_CHANGELOG

CMake 3.14 ships a module with compatible variables.

- Remove deprecated version checks
- Use VERSION_GREATER_EQUAL

CMake >= 3.0 supports bracket comments, and the reStructuredText
integration code in sphinx/ext/ecm.py already supports extracting
the docs from a bracket comment instead.
Editing documentation without leading line comment markers is more simple,
e,g. when reflowing text over lines.

With ECM meanwhile requiring CMake 3.5 now it is possible to switch
(and thus follow also the approach used by cmake itself).

NO_CHANGELOG

This is a slightly modernized version of Pino's original implementation
found in KF5 and Plasma.