class RbSys::CargoBuilder

def so_ext

Other tags:
    See: https://github.com/ruby/ruby/blob/c87c027f18c005460746a74c07cd80ee355b16e4/configure.ac#L3185 - 
def so_ext
  return RbConfig::CONFIG["SOEXT"] if RbConfig::CONFIG.key?("SOEXT")
  if win_target?
    "dll"
  elsif darwin_target?
    "dylib"
  else
    "so"
  end
end