Bundler issue with installing bcrypt-ruby

Bundle env

Bundler 1.3.1
Ruby 1.9.3 (2013-02-22 patchlevel 392) [x86_64-darwin12.2.1]
Rubygems 2.0.2
GEM_HOME 

Bundler settings
  path
    Set for your local app (/Users/sam/Sites/sample/.bundle/config): "./vendor/bundle"
  disable_shared_gems
    Set for your local app (/Users/sam/Sites/sample/.bundle/config): "1"


Gemfile
source "https://rubygems.org"
gem "sinatra"

gem "data_mapper"
gem "dm-postgres-adapter"


Gemfile.lock
GEM
  remote: https://rubygems.org/
  specs:
    rack (1.5.2)
    rack-protection (1.5.0)
      rack
    sinatra (1.4.1)
      rack (~> 1.5, >= 1.5.2)
      rack-protection (~> 1.4)
      tilt (~> 1.3, >= 1.3.4)
    tilt (1.3.6)

PLATFORMS
  ruby

DEPENDENCIES
  sinatra

Bundler issue.md

1. I am trying to accomplish installing data-mapper gem with the dm-postgres-adapter gem using bundler foir a sinatra app
2. I created a Gemfile (see Gemfile) and then in terminal ran bundle install
3. I expected all the gems to be downloaded and installed includign all dependency gems
4. What actually happened was when I ran bundle install was that when it got to installing bcrypt-ruby gem terminal ran into a build error (See Terminal error.md).

Gemfile

source "https://rubygems.org"
gem "sinatra"

gem "data_mapper"
gem "dm-postgres-adapter"

Terminal error.md

Installing bcrypt-ruby (3.0.1) 
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

/Users/sam/.rbenv/versions/1.9.3-p392/bin/ruby extconf.rb 
/Users/sam/.rbenv/versions/1.9.3-p392/bin/ruby: invalid option -R  (-h will show valid options) (RuntimeError)


Gem files will remain installed in /Users/sam/Sites/sample/vendor/bundle/ruby/1.9.1/gems/bcrypt-ruby-3.0.1 for inspection.
Results logged to /Users/sam/Sites/sample/vendor/bundle/ruby/1.9.1/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out

An error occurred while installing bcrypt-ruby (3.0.1), and Bundler cannot continue.
Make sure that `gem install bcrypt-ruby -v '3.0.1'` succeeds before bundling.