While attempting to install a package from gem, I encountered this issue:
sudo gem install dashing
Building native extensions. This could take a while...
ERROR: Error installing dashing:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from extconf.rb:2:in `'
Gem files will remain installed in /var/lib/gems/1.9.1/gems/eventmachine-1.2.3 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/eventmachine-1.2.3/ext/gem_make.out
Simply to solve this you just need to install ruby-dev packages on ubuntu:
sudo apt-get install ruby-dev