I was trying to install the rmagick gem and got the following error:

dev-01:~$ sudo gem install rmagick
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
checking for Ruby version >= 1.8.2… yes
checking for cc… yes
checking for …

etc.

This happens when the necessary libries are missing, I had installed the imagemagick package but this doesn’t include the dev libraries so I need to do :

dev-01:~$ sudo apt-get install libmagick++9-dev

after it completes, try again:

dev-01:~$ sudo gem install rmagick
Building native extensions.  This could take a while…
Successfully installed rmagick-2.7.1
1 gem installed

Which is what I wanted.