title: Nokogiri native extension failing to build (Not libxml2 or libxslt missing issue)
body: <p>As the title says, it doesn't seem to be failing because libxml2 or libxslt is missing.  I'm not really sure what to <em>make</em> of the error. (Get it? Because the issue is during make? hehe...)</p>
  
  <p>Anywho, here's the output I'm getting. Any ideas would be appreciated:</p>
  
  <pre><code>Building native extensions.  This could take a while...
  ERROR:  Error installing nokogiri:
      ERROR: Failed to build gem native extension.
  
          /usr/bin/ruby1.9.1 extconf.rb
  extconf.rb:10: Use RbConfig instead of obsolete and deprecated Config.
  checking for libxml/parser.h... yes
  checking for libxslt/xslt.h... yes
  checking for libexslt/exslt.h... yes
  checking for iconv_open() in iconv.h... yes
  checking for xmlParseDoc() in -lxml2... yes
  checking for xsltParseStylesheetDoc() in -lxslt... yes
  checking for exsltFuncRegister() in -lexslt... yes
  checking for xmlHasFeature()... yes
  checking for xmlFirstElementChild()... yes
  checking for xmlRelaxNGSetParserStructuredErrors()... yes
  checking for xmlRelaxNGSetParserStructuredErrors()... yes
  checking for xmlRelaxNGSetValidStructuredErrors()... yes
  checking for xmlSchemaSetValidStructuredErrors()... yes
  checking for xmlSchemaSetParserStructuredErrors()... yes
  creating Makefile
  
  make
  compiling xml_element_content.c
  compiling xml_relax_ng.c
  compiling xml_cdata.c
  compiling xml_sax_parser_context.c
  compiling xml_node_set.c
  compiling xml_text.c
  compiling xml_sax_parser.c
  compiling xml_attribute_decl.c
  compiling nokogiri.c
  nokogiri.c: In function ‘ruby_strdup’:
  nokogiri.c:43:31: warning: conversion to ‘size_t’ from ‘long int’ may change the sign of the result [-Wsign-conversion]
  compiling xml_entity_reference.c
  compiling xml_node.c
  compiling xml_xpath_context.c
  xml_xpath_context.c: In function ‘xpath_generic_exception_handler’:
  xml_xpath_context.c:184:3: error: format not a string literal and no format arguments [-Werror=format-security]
  cc1: some warnings being treated as errors
  make: *** [xml_xpath_context.o] Error 1
  
  
  Gem files will remain installed in /var/lib/gems/1.9.1/gems/nokogiri-1.5.0 for inspection.
  Results logged to /var/lib/gems/1.9.1/gems/nokogiri-1.5.0/ext/nokogiri/gem_make.out
  </code></pre>
  
  <p>For context, I upgraded from Ubuntu 12.04 to 12.10 last night. I had previously built the Nokogiri native extension on this box, though I do not know if it was the same version or not.</p>
  
  <p>I also checked that I have all packages specified in the second post on "<a href="https://stackoverflow.com/questions/8003523/error-installing-nokogiri-1-5-0-with-rails-3-1-0-and-ubuntu">error installing nokogiri 1.5.0 with rails 3.1.0 and ubuntu</a>"
   to no avail.</p>