Log message:
Add extra-cmake-modules version 5.18.0

Extra CMake Modules, or ECM, aims to augment CMake with additional modules.
It serves as both a staging ground for new modules before they are moved
upstream to CMake and a place for modules that, for whatever reason, are not
a good fit for CMake itself.

It is primarily driven by the needs of the KDE community, but it is hoped
that other projects will find it useful, and outside contributions are always
welcome. The main rule for new modules is that there must be at least two
downstream users.