问题如下：

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb extconf.rb:2:in `require': no such file to load -- mkmf (LoadError) from extconf.rb:2

我在 gem 里找不到 mkmf module，我用的是 ruby0-1.9.2 请问高人如何解决？