CI: add FreeBSD testing
It looks like Cirrus CI (https://cirrus-ci.org/guide/FreeBSD/) supports FreeBSD.
See here for more discussion:
Initially this can be done just for commits in master (say via GitHub), so that at least we know if we break something. FreeBSD is using Clang to build C++, so the C++ code itself will be fine, but it's more about ensuring CMake, linking as well as LLVM integration works well.