Mac を El Capitan にアップデートしたあとに gem がコケる件

一周遅れで El Capitan にアップデートしたところ、
以前はできていた gem のインストールができなかった。

$ sudo gem install jekyll-import

解決

下記の手順が必要だった。

1. Xcode コマンドラインツールをのインストール
   1. xcode-select --install
2. sudo gcc --version でライセンスに同意
   1. スペースキーでライセンスを読む
   2. agree と入力

---

困ってる人の検索用にエラーメッセージ。

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

    /Users/***/.rbenv/versions/2.1.2/bin/ruby extconf.rb
checking for ffi.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/***/.rbenv/versions/2.1.2/bin/ruby
    --with-ffi_c-dir
    --without-ffi_c-dir
    --with-ffi_c-include
    --without-ffi_c-include=${ffi_c-dir}/include
    --with-ffi_c-lib
    --without-ffi_c-lib=${ffi_c-dir}/lib
    --with-libffi-config
    --without-libffi-config
    --with-pkg-config
    --without-pkg-config
/Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:587:in `try_cpp'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:1067:in `block in have_header'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:918:in `block in checking_for'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:351:in `block (2 levels) in postpone'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:321:in `open'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:351:in `block in postpone'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:321:in `open'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:347:in `postpone'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:917:in `checking_for'
    from /Users/***/.rbenv/versions/2.1.2/lib/ruby/2.1.0/mkmf.rb:1066:in `have_header'
    from extconf.rb:16:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /Users/***/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ffi-1.9.14 for inspection.
Results logged to /Users/***/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/extensions/x86_64-darwin-14/2.1.0-static/ffi-1.9.14/gem_make.out