jekyll: Error installing - Failed to build gem native extension

This is everything that ran after trying to install jekyll:

Fetching: liquid-2.5.1.gem (100%)
Successfully installed liquid-2.5.1
Fetching: fast-stemmer-1.0.2.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing jekyll:
    ERROR: Failed to build gem native extension.

    /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h


Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/fast-stemmer-1.0.2 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/fast-stemmer-1.0.2/ext/gem_make.out