GIT_SILENT

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