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 AND

Currently, this means using build tool versions that are available for Ubuntu 16.04 (Xenial), Ubuntu 18.04 (Bionic), and Debian stretch.

Note, travis runs under Ubuntu 14.04 (Trusty) for linux builds.

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, the previous Ubuntu LTS release, as cmake3.