macOS Sierra
Ruby 2.3

gem install rails --no-ri --no-rdoc したところ以下のエラーで詰まる

Building native extensions.  This could take a while...
ERROR:  Error installing rails:
  ERROR: Failed to build gem native extension.

    current directory: /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/nio4r-1.2.1/ext/nio4r
/Users/{username}/.rbenv/versions/2.3.1/bin/ruby -r ./siteconf20161031-36453-o4rcff.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/{username}/.rbenv/versions/2.3.1/bin/$(RUBY_BASE_NAME)
/Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:587:in `try_cpp'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:1091:in `block in have_header'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:942:in `block in checking_for'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:350:in `block (2 levels) in postpone'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:320:in `open'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:350:in `block in postpone'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:320:in `open'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:346:in `postpone'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:941:in `checking_for'
  from /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/2.3.0/mkmf.rb:1090:in `have_header'
  from extconf.rb:3:in `<main>'

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

  /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/nio4r-1.2.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/nio4r-1.2.1 for inspection.
Results logged to /Users/{username}/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/nio4r-1.2.1/gem_make.out

やったこと

xcodeコマンドラインツールのインストール

xcode-select --install

gccのライセンスに同意

sudo gcc --version
↓
qで最下行表示後、"agree"を入力しEnter

再度実行

gem install rails --no-ri --no-rdoc

→無事インストールできた

参考: http://easyramble.com/failed-to-build-gem-native-extension.html