class Bundler::RubyGemsGemInstaller

def build_extensions
  extension_cache_path = options[:bundler_extension_cache_path]
  extension_dir = spec.extension_dir
  unless extension_cache_path && extension_dir
    prepare_extension_build(extension_dir)
    return super
  end
  build_complete = SharedHelpers.filesystem_access(extension_cache_path.join("gem.build_complete"), :read, &:file?)
  if build_complete && !options[:force]
    SharedHelpers.filesystem_access(File.dirname(extension_dir)) do |p|
      FileUtils.mkpath p
    end
    SharedHelpers.filesystem_access(extension_cache_path) do
      FileUtils.cp_r extension_cache_path, extension_dir
    end
  else
    prepare_extension_build(extension_dir)
    super
    SharedHelpers.filesystem_access(extension_cache_path.parent, &:mkpath)
    SharedHelpers.filesystem_access(extension_cache_path) do
      FileUtils.cp_r extension_dir, extension_cache_path
    end
  end
end