title: Gem unf_ext failed to build gem native extension
body: <p>env => Mac. OS Yosemite.</p>
  
  <p>I got a project use ruby 2.1.3 .
  But when I bundle install, there's an error I can't fix.
  the gem unf_ext have problem</p>
  
  <p>here's my terminal message:</p>
  
  <pre><code>$ gem install unf_ext -v '0.0.6'
  Building native extensions.  This could take a while...
  ERROR:  Error installing unf_ext:
  ERROR: Failed to build gem native extension.
  
  /Users/vincent/.rvm/rubies/ruby-2.1.3/bin/ruby extconf.rb
  checking for main() in -lstdc++... yes
  checking for ruby/encoding.h... yes
  creating Makefile
  
  make "DESTDIR=" clean
  
  make "DESTDIR="
  compiling unf.cc
  In file included from unf.cc:1:
  In file included from ./unf/normalizer.hh:4:
  In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/vector:265:
  In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/__bit_reference:15:
  In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:628:
  In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:604:
  /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iterator:341:10: fatal error: '__debug' file not found
  #include &lt;__debug&gt;
       ^
  1 error generated.
  make: *** [unf.o] Error 1
  
  make failed, exit code 2
  
  Gem files will remain installed in /Users/vincent/.rvm/gems/ruby-2.1.3/gems/unf_ext-0.0.6 for inspection.
  Results logged to /Users/vincent/.rvm/gems/ruby-2.1.3/extensions/x86_64-darwin-14/2.1.0-static/unf_ext-0.0.6/gem_make.out 
  </code></pre>
  
  <p>I try brew update and xcode select install
  but still not work.
  Any suggestion?</p>