一步一步(正确安装 ruby 和 rubygems)安装 watir-webdriver 后,我面临以下错误:
ERROR: Error installing watir-webdriver:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.9.1 extconf.rb
<internal:lib/rubygems/custom_require>:29:in `require': no such file to load -- mkmf (LoadError)
from <internal:lib/rubygems/custom_require>:29:in `require'
from extconf.rb:4:in `<main>'
请帮我解决这个问题,并让我知道是什么原因造成的?