[CMake] warn if features used that require >cmake-x.y

[Alexander Neundorf]

On Friday, July 03, 2015 09:33:42 Glenn Coombs wrote:
> I don't think policies are sufficient.  I just tried using the new
> target_sources command that was introduced in CMake 3.1.0 in a
> CMakeLists.txt file that specified cmake_minimum_required(VERSION 3.0) and
> it didn't warn that the CMakeLists.txt file wouldn't work with CMake 3.0.
> I think if you want to guarantee that it will work with CMake x.y then you
> must always use CMake x.y instead of using newer versions and hoping they
> will tell you if you have used something that won't work on the older
> version you require.

...or a nightly build using the minimum required versions of everything the 
project needs.

Alex