macOS Sierra での gem install nokogiri でエラー

以前にも同様のエントリを書きましたが、それと同様の手順だけではダメだったので新しく書きます。今回は Ruby 2.3.3 を新しくインストールした直後に生じたエラーです（他のバージョンでもインストールエラーが出ていたので、念のために Ruby 環境を新しくしました）。
前のヤツ：[Yosemite の gem install nokogiri でエラー（Xcode 7環境） - Qiita](http://qiita.com/riocampos/items/4a9da0017e64c8960bdf)

#環境
* macOS Sierra 10.12.3
* Ruby 2.3.3
* Nokogiri 1.7.0.1
* libxml2 2.9.4（Homebrew でインストール）

#結論
`brew install libxml2` と `brew link --force libxml2` （もしくは一気に `brew install --force libxml2`）してから `gem install nokogiri -- --use-system-libraries` する。

```bash
$ brew update
　：
$ brew install libxml2
　：
$ brew link --force libxml2
Linking /usr/local/Cellar/libxml2/2.9.4_1... 17 symlinks created
$ gem install nokogiri -- --use-system-libraries
Building native extensions with: '--use-system-libraries'
This could take a while...
Successfully installed nokogiri-1.7.0.1
1 gem installed
```
そして `libxml2` を unlink しておかないと `brew doctor` のときに怒られるので

```bash
$ brew unlink libxml2
```
しておきます [^1] 。
[^1]: 結局、 Homebrew で入れている `libxml2` の場所が見えないから Nokogiri が怒るんですよね。だからライブラリ位置指定してやれば良いのでしょうけど、やり方が分からない…orz
なお、 Nokogiri が必要とする `libxml2` のバージョンは Nokogiri のバージョン毎に違うはずので、 `gem install nokogiri -- --use-system-libraries` のために `libxml2` をインストールするならば、バージョン指定する必要が生じるときがあるかもしれません（調べてないです）。

参考リンク：[Mac OS X Mavericksで”gem i nokogiri”失敗の解決方法 - Qiita](http://qiita.com/t_732_twit/items/a7956a170b1694f7ffc2)
ただし参考リンクだと `libxslt` と `libiconv` のインストールも必要になってました。それまでの Homebrew 環境に依るのかも。

# 詳しい経緯と手順

## Ruby 2.3.3 インストール
```bash
$ CONFIGURE_OPTS="--disable-install-rdoc --enable-shared" rbenv install 2.3.3
ruby-build: use openssl from homebrew
Downloading ruby-2.3.3.tar.bz2...
-> https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.3.tar.bz2
Installing ruby-2.3.3...
ruby-build: use readline from homebrew
Installed ruby-2.3.3 to /Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3

Communized gems for 2.3.3
$ rbenv global 2.3.3
```
参考：[rbenvでMacにRuby 2.2.1 インストール（オプション指定最新版 - Qiita](http://qiita.com/riocampos/items/0df10dd7956ce9de0ffb)
## Nokogiri インストールエラー
```
$ gem install nokogiri
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
	ERROR: Failed to build gem native extension.

    current directory: /Users/riocampos/.rbenv/gems/2.3.0/gems/nokogiri-1.7.0.1/ext/nokogiri
/Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3/bin/ruby -r ./siteconf20170224-8319-1hb44k6.rb extconf.rb
checking if the C compiler accepts ... yes
checking if the C compiler accepts -Wno-error=unused-command-line-argument-hard-error-in-future... no
Building nokogiri using packaged libraries.
Using mini_portile version 2.1.0
checking for iconv.h... yes
checking for gzdopen() in -lz... yes
checking for iconv... yes
************************************************************************
IMPORTANT NOTICE:

Building Nokogiri with a packaged version of libxml2-2.9.4.

Team Nokogiri will keep on doing their best to provide security
updates in a timely manner, but if this is a concern for you and want
to use the system library instead; abort this installation process and
reinstall nokogiri as follows:

    gem install nokogiri -- --use-system-libraries
        [--with-xml2-config=/path/to/xml2-config]
        [--with-xslt-config=/path/to/xslt-config]

If you are using Bundler, tell it to use the option:

    bundle config build.nokogiri --use-system-libraries
    bundle install

Note, however, that nokogiri is not fully compatible with arbitrary
versions of libxml2 provided by OS/package vendors.
************************************************************************
Extracting libxml2-2.9.4.tar.gz into tmp/x86_64-apple-darwin16.4.0/ports/libxml2/2.9.4... OK
Running 'configure' for libxml2 2.9.4... OK
Running 'compile' for libxml2 2.9.4... ERROR, review '/Users/riocampos/.rbenv/gems/2.3.0/gems/nokogiri-1.7.0.1/ext/nokogiri/tmp/x86_64-apple-darwin16.4.0/ports/libxml2/2.9.4/compile.log' to see what happened. Last lines are:
========================================================================
    unsigned short* in = (unsigned short*) inb;
                         ^~~~~~~~~~~~~~~~~~~~~
encoding.c:815:27: warning: cast from 'unsigned char *' to 'unsigned short *' increases required alignment from 1 to 2 [-Wcast-align]
    unsigned short* out = (unsigned short*) outb;
                          ^~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.
  CC       error.lo
  CC       parserInternals.lo
  CC       parser.lo
  CC       tree.lo
  CC       hash.lo
  CC       list.lo
  CC       xmlIO.lo
xmlIO.c:1450:52: error: use of undeclared identifier 'LZMA_OK'
    ret =  (__libxml2_xzclose((xzFile) context) == LZMA_OK ) ? 0 : -1;
                                                   ^
1 error generated.
make[2]: *** [xmlIO.lo] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
========================================================================
*** 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/riocampos/.anyenv/envs/rbenv/versions/2.3.3/bin/$(RUBY_BASE_NAME)
	--help
	--clean
	--use-system-libraries
	--enable-static
	--disable-static
	--with-zlib-dir
	--without-zlib-dir
	--with-zlib-include
	--without-zlib-include=${zlib-dir}/include
	--with-zlib-lib
	--without-zlib-lib=${zlib-dir}/lib
	--enable-cross-build
	--disable-cross-build
/Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:366:in `block in execute': Failed to complete compile task (RuntimeError)
	from /Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:337:in `chdir'
	from /Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:337:in `execute'
	from /Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:111:in `compile'
	from /Users/riocampos/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:150:in `cook'
	from extconf.rb:364:in `block (2 levels) in process_recipe'
	from extconf.rb:257:in `block in chdir_for_build'
	from extconf.rb:256:in `chdir'
	from extconf.rb:256:in `chdir_for_build'
	from extconf.rb:363:in `block in process_recipe'
	from extconf.rb:262:in `tap'
	from extconf.rb:262:in `process_recipe'
	from extconf.rb:547:in `<main>'

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

  /Users/riocampos/.anyenv/envs/rbenv/gems/2.3.0/extensions/x86_64-darwin-16/2.3.0/nokogiri-1.7.0.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/riocampos/.anyenv/envs/rbenv/gems/2.3.0/gems/nokogiri-1.7.0.1 for inspection.
Results logged to /Users/riocampos/.anyenv/envs/rbenv/gems/2.3.0/extensions/x86_64-darwin-16/2.3.0/nokogiri-1.7.0.1/gem_make.out
```
そして、よくある解決法の `$ gem install nokogiri -- --use-system-libraries` に頼ったのですが、やはり同様のエラーが生じました。

## `brew install libxml2` のときの注意書き
```bash
$ brew install libxml2
Updating Homebrew...
Warning: libxml2 is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
```
リンクしてない（Homebrew でインストールした `libxml2` を強いては使わない設定）ので `gem install nokogiri -- --use-system-libraries` のときに、この `libxml2` を見てくれなかったのでしょうね。