Make the default build type "Debug" when compiling a git checkout.

Summary:
The idea comes from
https://blog.kitware.com/cmake-and-the-default-build-type/
but I adapted it to leave it empty for tarballs, to avoid messing up
distribution packaging.

The goal is to make this more sensible for [new] developers who just
run cmake and end up with a "no debug symbols, no optimizations" build,
i.e. the one and only completely useless combination of those two flags.

Possible risk: distributions who compile from git checkouts...

Test Plan: "mkdir build ; cd build ; cmake .." in kblog leads to CMAKE_BUILD_TYPE=Debug

Reviewers: kde-buildsystem, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, apol, kde-frameworks-devel

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D22667