jruby で native extension がインストール出来ない問題

gistfile1.txt

$ gem install fog
Fetching: builder-3.2.2.gem (100%)
Successfully installed builder-3.2.2
Fetching: excon-0.27.6.gem (100%)
Successfully installed excon-0.27.6
Fetching: formatador-0.2.4.gem (100%)
Successfully installed formatador-0.2.4
Fetching: multi_json-1.8.2.gem (100%)
Successfully installed multi_json-1.8.2
Fetching: mime-types-1.25.gem (100%)
Successfully installed mime-types-1.25
Fetching: net-ssh-2.7.0.gem (100%)
Successfully installed net-ssh-2.7.0
Fetching: net-scp-1.1.2.gem (100%)
Successfully installed net-scp-1.1.2
Fetching: mini_portile-0.5.1.gem (100%)
Successfully installed mini_portile-0.5.1
Fetching: nokogiri-1.6.0-java.gem (100%)
Successfully installed nokogiri-1.6.0-java
Fetching: ruby-hmac-0.4.0.gem (100%)
Successfully installed ruby-hmac-0.4.0
Fetching: unicode-0.4.4.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing fog:
        ERROR: Failed to build gem native extension.

    /home/thirai/.rvm/rubies/jruby-1.7.5/bin/jruby extconf.rb
NotImplementedError: C extension support is not enabled. Pass -Xcext.enabled=true to JRuby or set JRUBY_OPTS.

   (root) at /home/thirai/.rvm/rubies/jruby-1.7.5/lib/ruby/shared/mkmf.rb:8
  require at org/jruby/RubyKernel.java:1082
   (root) at /home/thirai/.rvm/rubies/jruby-1.7.5/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:1
   (root) at extconf.rb:1


Gem files will remain installed in /home/thirai/.rvm/gems/jruby-1.7.5/gems/unicode-0.4.4 for inspection.
Results logged to /home/thirai/.rvm/gems/jruby-1.7.5/gems/unicode-0.4.4/ext/unicode/gem_make.out