Error installing rmagick, Failed to build gem native extension, no such file to load -- mkmf (LoadError)

ubuntu@desktop:~$ sudo gem install rmagick
[sudo] password for ubuntu:
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

Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/rmagick-2.9.1 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/rmagick-2.9.1/ext/RMagick/gem_make.out


SOLUTION

• sudo apt-get install ruby1.8-dev # Install this first.
  
• sudo gem install rmagick