>> sudo gem install god
Building native extensions. This could take a while...
ERROR: Error installing god:
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:1:in `<main>'
Gem files will remain installed in /var/lib/gems/1.9.1/gems/god-0.13.5 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/god-0.13.5/ext/god/gem_make.out
The problem is fixed by installing ruby-dev, as mentioned in this post:
http://stackoverflow.com/questions/7645918/require-no-such-file-to-load-mkmf-loaderror.