no such file to load — mkmf

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Building native extensions. This could take a while...
  
ERROR: Error installing rails:
      
ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb
  
extconf.rb:1:in \`require': no such file to load - mkmf (LoadError)
      
from extconf.rb:1

For some reason, mkmf.rb is part of the ruby1.8-dev package, and initially I hadn’t installed that.

#install ruby1.8-dev

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sudo apt-get install ruby1.8-dev

and everything trotted along happily after that.