Adopt a policy for the minimum version of main dependencies
Right now, on Linux we're saying Wt needs at least GCC 5, Boost 1.50.0, and CMake 3.1. On Windows we require Visual Studio 2015. This is because at the time these were the earliest versions I could find that supported Wt 4 fully.
While solid backwards compatibility is one of Wt's goals, it's not realistic to have to continue to support very old versions of our main dependencies.
I therefore propose that we adapt a clear policy for how for back we want to go. This will make it clear to users and customers what they can expect, and it makes it so we don't need to have a discussion every time we decide to bump the minimum requirements.
One possible approach would be to say: we're going back to the earliest Ubuntu LTS that's still supported or the oldest Debian version that's still supported and look at its compiler and library versions. This would be something like a 5 year sliding window.