macでnokogiriがインストールできない対応

# 確認環境

* macOS Sierra: 10.12.3
* Ruby: 2.4.1 (rbenvでインストール)
* nokogiri: 1.7.1
* **他の環境でもこの手順でコケた記憶がないので環境が違ってもいけるはず**

## 対応方法

```
brew tap homebrew/dupes
brew install libxml2 libxslt libiconv
brew link libxml2 --force
brew link libxslt --force
gem install nokogiri -- --use-system-libraries --with-iconv-dir="$(brew --prefix libiconv)" --with-xml2-config="$(brew --prefix libxml2)/bin/xml2-config" --with-xslt-config="$(brew --prefix libxslt)/bin/xslt-config"
```

誰かのエラーを解決できればと思います :grinning:


# 下記のよくあるエラーを対応

```
gem install nokogiri                                                                                                                                                   (git)-[master] ? 
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
  ERROR: Failed to build gem native extension.

    current directory: /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nokogiri-1.7.1/ext/nokogiri
/Users/nipe/.rbenv/versions/2.4.1/bin/ruby -r ./siteconf20170406-61643-1q52fpv.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
with the following patches applied:
  - 0001-Fix-comparison-with-root-node-in-xmlXPathCmpNodes.patch
  - 0002-Fix-XPointer-paths-beginning-with-range-to.patch
  - 0003-Disallow-namespace-nodes-in-XPointer-ranges.patch

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 git apply with /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nokogiri-1.7.1/patches/libxml2/0001-Fix-comparison-with-root-node-in-xmlXPathCmpNodes.patch... OK
Running git apply with /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nokogiri-1.7.1/patches/libxml2/0002-Fix-XPointer-paths-beginning-with-range-to.patch... OK
Running git apply with /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nokogiri-1.7.1/patches/libxml2/0003-Disallow-namespace-nodes-in-XPointer-ranges.patch... OK
Running 'configure' for libxml2 2.9.4... OK
Running 'compile' for libxml2 2.9.4... ERROR, review '/Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nokogiri-1.7.1/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/nipe/.rbenv/versions/2.4.1/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/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/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/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:337:in `chdir'
  from /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:337:in `execute'
  from /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:111:in `compile'
  from /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/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/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0-static/nokogiri-1.7.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/nokogiri-1.7.1 for inspection.
Results logged to /Users/nipe/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0-static/nokogiri-1.7.1/gem_make.out
```