build_extensions

# File lib/bundler/rubygems_gem_installer.rb, line 21
    def build_extensions
      extension_cache_path = options[:bundler_extension_cache_path]
      return super unless extension_cache_path && extension_dir = Bundler.rubygems.spec_extension_dir(spec)

      extension_dir = Pathname.new(extension_dir)
      build_complete = SharedHelpers.filesystem_access(extension_cache_path.join("gem.build_complete"), :read, &:file?)
      if build_complete && !options[:force]
        SharedHelpers.filesystem_access(extension_dir.parent, &:mkpath)
        SharedHelpers.filesystem_access(extension_cache_path) do
          FileUtils.cp_r extension_cache_path, spec.extension_dir
        end
      else
        super
        if extension_dir.directory? # not made for gems without extensions
          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
    end