Consider using ninja to build CMake projects

Currently BuildStream uses make to build CMake projects. CMake has an "experimental" ninja backend, which I put in quotes because it has been quite stable for many years and is totally superior to the make backend. It probably does not make much difference for small projects or clean builds, but for incremental builds of large projects, ninja is dramatically faster. That's important for developers using BuildStream workspaces.

BuildStream already uses ninja to build meson projects, so everything should be exactly the same except for the call meson/cmake itself. Just use cmake -GNinja to tell CMake to generate a ninja build file instead of makefiles.