Estoy tratando de instalar newgem en mi máquina Linux (sudo gem install newgem) y estoy recibiendo el siguiente error:Error al instalar en Linux newgem
Building native extensions. This could take a while...
ERROR: Error installing newgem:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.8 extconf.rb install newgem
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/RedCloth-4.0.4 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/RedCloth-4.0.4/ext/redcloth_scan/gem_make.out
Cuál podría ser el problema?