R – Error in webrat installation

I'm getting the following error when I'm trying to install webrat in my OS X, please suggest me how can i solve this problem.

ERROR:

Error installing webrat:
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/nokogiri-1.4.0 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0/ext/nokogiri/gem_make.out