Some fixes for CMakeLists.txt

Summary:

• bump cmake dep to 3.0, spares one policy (any target group for 2.8?)
• do cmake_minimum_required as first thing
• use ECM things only if ECM_FOUND
• do things covered by KDE{Compiler,CMake}Settings only if !ECM
• install the generator actually :)

Reviewers: vandenoever

Reviewed By: vandenoever

Differential Revision: https://phabricator.kde.org/D9357