Fix build with pre-release libssh in Ubuntu 18.04, add libssh to CMake #5372
Ubuntu 18.04 is the only known distro version that ships with a pre-release version of libssh. Since they are unlikely to upgrade their libssh version, add a workaround to prevent build failures due to deprecation errors. This change is a dependency for #5370.
CMake support was added so I could easily test the libssh build.
This is not documented anywhere (blog, changelog, docs), but indeed 20.04 support appears to be work in progress in the travis-build, docs-travis-ci-com, and travis-yml repos (search for "focal"). If that is available, I'll drop the version check here!