# File lib/rubygems/installer.rb, line 146
    def build_support_directories(install_dir)
       unless File.exist? File.join(install_dir, "specifications")
         FileUtils.mkdir_p File.join(install_dir, "specifications")
       end
       unless File.exist? File.join(install_dir, "cache")
         FileUtils.mkdir_p File.join(install_dir, "cache")
       end
       unless File.exist? File.join(install_dir, "doc")
         FileUtils.mkdir_p File.join(install_dir, "doc")
       end
    end