Build tool dependency policy

To ensure the broadest compatibility when building the benchmark library, but still allow forward progress, we require any build tooling to be available for:

• Debian stable and
• The last two Ubuntu LTS releases

Currently, this means using build tool versions that are available for Ubuntu 18.04 (Bionic Beaver), Ubuntu 20.04 (Focal Fossa), and Debian 11 (bullseye).

Note, CI also runs ubuntu-16.04 and ubuntu-14.04 to ensure best effort support for older versions.

cmake

The current supported version is cmake 3.5.1 as of 2018-06-06.

Note, this version is also available for Ubuntu 14.04, an older Ubuntu LTS release, as cmake3.