# File lib/librarian/posix.rb, line 16
      def which(cmd)
        exts = ENV["PATHEXT"] ? ENV["PATHEXT"].split(';') : ['']
        ENV["PATH"].split(File::PATH_SEPARATOR).each do |path|
          path = File.expand_path(path)
          exts.each do |ext|
            exe = File.join(path, cmd + ext)
            return exe if File.file?(exe) && File.executable?(exe)
          end
        end
        nil
      end