当我尝试在ubuntu上安装rails时出现此错误：
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from extconf.rb:1:in `<main>'
Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.7.5 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.7.5/ext/json/ext/generator/gem_make.out
我用rvm安装。我不知道为什么会一直发生