Updated CMake

This MR updates the minimum CMake version required to 3.10, which is supported by both Ubuntu 18 (3.10 supported till April 23) and and Debian Buster (3.13 EOL August 22, LTS?). It is not included in Debian stretch, which is EOL but still receives LTS until June 22. I have also removed the option to disable C++11 tests from the CMake file, and cleaned up the corresponing names in the CI.

I have also included a second commit which changes the minimum version of gcc to GCC 5. This might conflict or be redundant with !739 (merged), so if you like I can also submit a PR without the second change.