There is a chance it's going to be used in Solid, and possibly KIO.

Other exiting modules were used as templates while writing this.

CMake 3.14 ships a module with compatible variables.

- Remove deprecated version checks
- Use VERSION_GREATER_EQUAL

pkg_search_module() is only useful when the pkgconfig file may have different names.

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

The code argument has been deprecated in favor of public-code and
private-code options.

In general, it is highly advised to use the private-code option, but we
cannot do it because it will break the ABI.

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

We have identical copies of this in kinit, kwin, ksysguard and
powerdevil. This makes it a natural candidate for inclusion in ECM.