Installation fail on Ubuntu 14.04

[BuonOmo]

Installing with gem (version 2.6.12) for ruby 2.0.0p384 gives the next message:

current directory: /var/lib/gems/1.9.1/gems/yajl-ruby-1.3.0/ext/yajl
/usr/bin/ruby1.9.1 -r ./siteconf20170518-21166-juwweo.rb extconf.rb
/usr/local/lib/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/local/lib/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:55:in `require'
	from extconf.rb:1:in `<main>'

extconf failed, exit code 1