We need ninja-build to use the cmake+ninja buildsystem.
Just refactoring. This does not change the behavior. This is the common style, and it makes it more explicit which build system is in use.
Fix GH-35 --------- Co-authored-by: otegami <otegami@clear-code.com> Co-authored-by: Sutou Kouhei <kou@clear-code.com>