no such file to load — mkmf

shetty@shetty-laptop:~$ sudo gem install rmagick
[sudo] password for shetty: 
Building native extensions.  This could take a while...
ERROR:  Error installing rmagick:
	ERROR: Failed to build gem native extension.

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

 mkmf.rb 是ruby1.8-dev package 的一部分，安装ruby1.8-dev就可以了

sudo apt-get install ruby1.8-dev