BundlerでC拡張を含んだgemを公開する ref: http://qiita.com/gam0022/items/2ee82e84e5c9f608eb85

extconf.rb

require "mkmf"
create_makefile("immutable_list/immutable_list")

file0.txt

$ bundle gem immutable_list
      create  immutable_list/Gemfile
      create  immutable_list/Rakefile
      create  immutable_list/LICENSE.txt
      create  immutable_list/README.md
      create  immutable_list/.gitignore
      create  immutable_list/immutable_list.gemspec
      create  immutable_list/lib/immutable_list.rb
      create  immutable_list/lib/immutable_list/version.rb
Initializating git repo in /Users/gam0022/git/gem/t/immutable_list

file4.txt

ext
└── immutable_list
    ├── extconf.rb
    └── immutable_list.c

file7.txt

lib
├── immutable_list
│   └── version.rb
└── immutable_list.rb

Gemfile

 source 'https://rubygems.org'

 # Specify your gem's dependencies in immutable_list.gemspec
 gemspec
+gem "rake-compiler"

immutable_list.c

#include <stdio.h>
#include <ruby.h>

#define true 1
#define false 0

VALUE cImmutableList;

struct immutable_list {
  VALUE value;
  VALUE next;
};

static void
immutable_list_mark(struct immutable_list *ptr)
{
  rb_gc_mark(ptr->value);
  rb_gc_mark(ptr->next);
}

// 長すぎるので以下略。気になる人は以下を参照
// https://github.com/gam0022/immutable_list/blob/master/ext/immutable_list/immutable_list.c

immutable_list.gemspec

# coding: utf-8
 lib = File.expand_path('../lib', __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'immutable_list/version'

 Gem::Specification.new do |spec|
   spec.name          = "immutable_list"
   spec.version       = ImmutableList::VERSION
   spec.authors       = ["gam0022"]
   spec.email         = ["[email protected]"]
-  spec.description   = %q{TODO: Write a gem description}
-  spec.summary       = %q{TODO: Write a gem summary}
+  spec.description   = %q{Immutable Linked List implemented in C-Extensions}
+  spec.summary       = %q{Immutable Linked List implemented in C-Extensions}
   spec.homepage      = ""
   spec.license       = "MIT"
+  spec.extensions    = %w[ext/immutable_list/extconf.rb]

   spec.files         = `git ls-files`.split($/)
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]

   spec.add_development_dependency "bundler", "~> 1.3"
   spec.add_development_dependency "rake"
 end

immutable_list.rb

 require "immutable_list/version"
+require "immutable_list/immutable_list"

-module ImmutableList
+class ImmutableList
   # Your code goes here...
 end

Rakefile

 require "bundler/gem_tasks"
+require "rake/extensiontask"
+
+Rake::ExtensionTask.new "immutable_list" do |ext|
+  ext.lib_dir = "lib/immutable_list"
+end

version.rb

-module ImmutableList
+class ImmutableList
  VERSION = "0.0.1"
end