`require': cannot load such file -- mkmf (LoadError)

Gem::Installer::ExtensionBuildError: 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/RedCloth-4.2.9 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/RedCloth-4.2.9/ext/redcloth_scan/gem_make.out

An error occurred while installing RedCloth (4.2.9), and Bundler cannot continue.
Make sure that `gem install RedCloth -v '4.2.9'` succeeds before bundling.

$ sudo apt-get install ruby1.9.1-dev

$ locate mkmf
~/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/mkmf.rb
~/.rbenv/versions/1.9.3-p194/share/ri/1.9.1/system/Object/mkmf-i.ri