/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)

[mherweg]

gem install pssh
Fetching: json-1.8.2.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing pssh:
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:inrequire'
from extconf.rb:1:in `

'

Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.2 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.8.2/ext/json/ext/generator/gem_make.out