$ sudo gem install cassandra
Building native extensions.  This could take a while...
ERROR:  Error installing cassandra:
	ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb
extconf.rb:20:in `require': no such file to load -- mkmf (LoadError)
	from extconf.rb:20


Gem files will remain installed in /var/lib/gems/1.8/gems/thrift-0.5.0 for inspection.
Results logged to /var/lib/gems/1.8/gems/thrift-0.5.0/ext/gem_make.out