インストールしようとしhpricotていますが、次のエラーが表示されます。xcode はインストールされていますが、他に何が欠けているのかわかりません。
$ gem install hpricot
Building native extensions.  This could take a while...
ERROR:  Error installing hpricot:
    ERROR: Failed to build gem native extension.
        /Users/user_name/.rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
checking for stdio.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.
のこぎりもやってみました。
$ gem install nokogiri
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
    ERROR: Failed to build gem native extension.
        /Users/user_name/.rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
checking for libxml/parser.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.