New cmake warning.

[Kevin Kofler]

Méven wrote:
> I am guessing you are using cmake >= 3.16 so it allows you to run, but
> since the project does not have a cmake minimum, for others using cmake <
> 3.16 the build will break.
> This warning highlights it, so you inform your contributor they need cmake
> 3.16 before running into the hard fail in FindKF5.
> 
> TLDR; If you use FindKF5,  you should add cmake minimal version 3.16.

Well, this check is broken because older ECM will work just fine with older 
CMake. There are versions out there that accept as far back as CMake 2.8.12. 
If the project does not require the latest versions of ECM and KF5, it does 
not necessarily have to require their current minimum version of CMake 
either.

        Kevin Kofler