Move package version definition to top level dir, which is common practice

Summary:
CMake, starting with version 3.0.2, has support to specify project
version through the cmake project command in top level dir, which is
used by this commit.

With cmake > 2.8.13, the required "extra cmake modules" package
suggests using cmake's config helpers module instead of the one
provided by ECM.

Test Plan: compiled on linux

Reviewers: tbaumgart

Reviewed By: tbaumgart

Differential Revision: https://phabricator.kde.org/D11277