Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby -r ./siteconf20150404-6-1pa98vy.rb extconf.rb
/usr/lib/ruby/2.2/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/lib/ruby/2.2/rubygems/core_ext/kernel_require.rb:54:in `require'
	from extconf.rb:1:in `<main>'

extconf failed, exit code 1