Building native extensions.  This could take a while...
ERROR:  Error installing json:
	ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.8 extconf.rb
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
	from extconf.rb:1


Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/json-1.6.5 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/json-1.6.5/ext/json/ext/parser/gem_make.out