How to add Mac-specific gems to bundle on Mac but not on Linux?

I'm developing a Rails app on a Mac, and I'm new to testing, so I just added these gems to my Gemfile:

group :test, :development do
  gem 'rspec-rails'     
  gem 'rb-fsevent'
  gem 'growl'
end

But my production server runs Linux, so even if they aren't grouped in :production, bundler (v1.0.21) still attempts to install them. ...and fails, obviously!
extconf.rb:19:in '<main>': Only Darwin (Mac OS X) systems are supported (RuntimeError)

Setting RAILS_ENV to production before running bundle install doesn't work.

It worked by running bundle install --without development test, but how can these gems be taken into consideration by bundler only based on your OS?

Edit: The bundler wiki provides details on how to use platform as a parameter.
The same solution is given in bundler issue #663, so I tried:

group :test, :development do
  gem 'rspec-rails'     
  platforms :darwin do
    gem 'rb-fsevent'
    gem 'growl'
  end 
end

bundle install does not work, but even if we go back to square one and do
bundle install --without darwin, the result is 'darwin' is not a valid platform.
The available options are: [:ruby, :ruby_18, :ruby_19, :mri, :mri_18, :mri_19, :rbx, :jruby, :mswin, :mingw, :mingw_18, :mingw_19]

Any other (elegant) approaches?