require cmake 3.8 and declare the version into a single place

Summary:
to allow simpler management of the version, only declare it in
CMakeLists.txt project command and use that to set it in the code

require the minimal version needed to get c++17

Test Plan: tests are OK

Reviewers: Elisa, rikmills, ngraham

Reviewed By: Elisa, ngraham

Subscribers: pino

Differential Revision: https://phabricator.kde.org/D21404