title: ERROR: Failed to build gem native extension
body: <pre><code>gem install unf_ext -v '0.0.6'
  localhost:workflow_boyun ryan007$ 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/ryan007/.rvm/rubies/ruby-1.9.3-p392/bin/ruby extconf.rb
  checking for main() in -lstdc++... *** 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.
  
  Provided configuration options:
      --with-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/bin/ruby
      --with-stdc++lib
      --without-stdc++lib
  /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
  You have to install development tools first.
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:461:in `try_link0'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:476:in `try_link'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:619:in `try_func'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:845:in `block in have_library'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:254:in `open'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:254:in `open'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
      from /Users/ryan007/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:840:in `have_library'
      from extconf.rb:2:in `&lt;main&gt;'
  
  
  Gem files will remain installed in /Users/ryan007/.rvm/gems/ruby-1.9.3-p392@rails3.2.8/gems/unf_ext-0.0.6 for inspection.
  Results logged to /Users/ryan007/.rvm/gems/ruby-1.9.3-p392@rails3.2.8/gems/unf_ext-0.0.6/ext/unf_ext/gem_make.out
  </code></pre>