I've seen Guard and it's usefulness in web development and would like to install it. Naturally, I expected to be able to use sudo gem install guard to do so (yes, it is a Ruby Gem). However the installation errors out. Here is what I'm getting:

Building native extensions.  This could take a while...

ERROR:  Error installing guard:

    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:4:in `<main>'





Gem files will remain installed in /var/lib/gems/1.9.1/gems/ffi-1.9.3 for inspection.

Results logged to /var/lib/gems/1.9.1/gems/ffi-1.9.3/ext/ffi_c/gem_make.out

Does anyone know how I can fix this? Thanks for any help.