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
2. xcode-select --install
3. sudo gcc --version でライセンスに同意
4. スペースキーでライセンスを読む
5. 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