MacでRails5のbundle install時にnio4rのエラーが出る #Ruby

macOS Sierra
Ruby 2.4.1

Rails5をインストールしようとしたら、nio4rのとこでエラーが出る。

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
    current directory: /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nio4r-2.1.0/ext/nio4r
/Users/ryo/.rbenv/versions/2.4.1/bin/ruby -r ./siteconf20171009-1873-ylw7vp.rb extconf.rb
checking for unistd.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/Users/ryo/.rbenv/versions/2.4.1/bin/$(RUBY_BASE_NAME)
/Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:457:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:588:in `try_cpp'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:1095:in `block in have_header'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:945:in `block in checking_for'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:351:in `block (2 levels) in postpone'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:321:in `open'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:351:in `block in postpone'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:321:in `open'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:347:in `postpone'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:944:in `checking_for'
    from /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/2.4.0/mkmf.rb:1094:in `have_header'
    from extconf.rb:5:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0-static/nio4r-2.1.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nio4r-2.1.0 for inspection.
Results logged to /Users/ryo/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0-static/nio4r-2.1.0/gem_make.out

An error occurred while installing nio4r (2.1.0), and Bundler cannot continue.
Make sure that `gem install nio4r -v '2.1.0'` succeeds before bundling.

In Gemfile:
  rails was resolved to 5.1.4, which depends on
    actioncable was resolved to 5.1.4, which depends on
      nio4r

書いてある通り、 mkmf.log の中身を見てみたら以下の1行が書いてある。

Agreeing to the Xcode/iOS license requires admin privileges, please re-run as root via sudo.

Xcode9.0にバージョンアップした為か、ライセンスに同意してない状態だった為にエラーになっていたようでした。
Xcodeを起動して、ライセンス画面でagreeを選択したらエラー解消されました。