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...