OSX への nokogiri 1.6.8.rc3 の install でハマって解決したメモ

## 最初
普通に `bundle install` したら、以下のエラーでこけた。
libxml2-2.9.3 の compile に失敗してる。変数 `LZMA_OK` が無いとかいうエラー。。。
http://louistiao.me/posts/installing-lxml-on-mac-osx-1011-inside-a-virtualenv-with-pip/ では、（pip だから文脈は違うけど）OSX では libxml2-2.9.3 じゃない version を使えって書いてる。


```shell
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/nokogiri-1.6.8.rc3/ext/nokogiri
/Users/[user]/.rbenv/versions/2.3.0/bin/ruby -r ./siteconf20160304-52550-ujlj25.rb extconf.rb
Using pkg-config version 1.1.7
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 libxml2-2.9.3.

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 libxml2-2.9.3.tar.gz into tmp/x86_64-apple-darwin15.3.0/ports/libxml2/2.9.3... OK
Running 'configure' for libxml2 2.9.3... OK
Running 'compile' for libxml2 2.9.3... ERROR, review '/Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/nokogiri-1.6.8.rc3/ext/nokogiri/tmp/x86_64-apple-darwin15.3.0/ports/libxml2/2.9.3/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/[user]/.rbenv/versions/2.3.0/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/[user]/.rbenv/versions/2.3.0/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/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:337:in `chdir'
        from /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:337:in `execute'
        from /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:111:in `compile'
        from /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/mini_portile2-2.1.0/lib/mini_portile2/mini_portile.rb:150:in `cook'
        from extconf.rb:328:in `block (2 levels) in process_recipe'
        from extconf.rb:221:in `block in chdir_for_build'
        from extconf.rb:220:in `chdir'
        from extconf.rb:220:in `chdir_for_build'
        from extconf.rb:327:in `block in process_recipe'
        from extconf.rb:226:in `tap'
        from extconf.rb:226:in `process_recipe'
        from extconf.rb:507:in `<main>'

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

  /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/nokogiri-1.6.8.rc3/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/nokogiri-1.6.8.rc3 for inspection.
Results logged to /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/nokogiri-1.6.8.rc3/gem_make.out
```

## system の libxml2 を使って解決
nokogiri に利用する libxml2 の指定を任せると 2.9.3 を compile しようとしてコケちゃうみたいなので、homebrew で pkg-config を入れてその中の `libxmll2` を利用する事にした。ちなみに実際に利用したのは `libxml2-2.9.1`。

上記で出たエラーメッセージに従って、`bundle install` 時に system の libxml2 を使うという設定をする

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

設定値は、 `~/.bundle/config` から確認出来る。

```~/.bundle/config
---
BUNDLE_GEM__COC: 'false'
BUNDLE_GEM__MIT: 'true'
BUNDLE_GEM__TEST: 'false'
BUNDLE_BUILD__NOKOGIRI: "--use-system-libraries"
```

この状態で再び `bundle install`。またエラーが出る。

```
Installing nokogiri 1.6.8.rc3 with native extensions

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/nokogiri-1.6.8.rc3/ext/nokogiri
/Users/[user]/.rbenv/versions/2.3.0/bin/ruby -r ./siteconf20160304-57103-8nd8hm.rb extconf.rb --use-system-libraries
Using pkg-config version 1.1.7
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 system libraries.
ERROR: cannot discover where libxml2 is located on your system. please make sure `pkg-config` is installed.
*** 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/[user]/.rbenv/versions/2.3.0/bin/$(RUBY_BASE_NAME)
        --help
        --clean
        --use-system-libraries
        --with-zlib-dir
        --without-zlib-dir
        --with-zlib-include
        --without-zlib-include=${zlib-dir}/include
        --with-zlib-lib
        --without-zlib-lib=${zlib-dir}/lib
        --with-xml2-dir
        --without-xml2-dir
        --with-xml2-include
        --without-xml2-include=${xml2-dir}/include
        --with-xml2-lib
        --without-xml2-lib=${xml2-dir}/lib
        --with-libxml-2.0-config
        --without-libxml-2.0-config
        --with-pkg-config
        --without-pkg-config
        --with-xslt-dir
        --without-xslt-dir
        --with-xslt-include
        --without-xslt-include=${xslt-dir}/include
        --with-xslt-lib
        --without-xslt-lib=${xslt-dir}/lib
        --with-libxslt-config
        --without-libxslt-config
        --with-exslt-dir
        --without-exslt-dir
        --with-exslt-include
        --without-exslt-include=${exslt-dir}/include
        --with-exslt-lib
        --without-exslt-lib=${exslt-dir}/lib
        --with-libexslt-config
        --without-libexslt-config

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

  /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/nokogiri-1.6.8.rc3/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/nokogiri-1.6.8.rc3 for inspection.
Results logged to /Users/[user]/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/nokogiri-1.6.8.rc3/gem_make.out
```

以下の様なエラーメッセージが出てて、どうもリンカが `libxml2` を見つけられてないっぽい。

```
ERROR: cannot discover where libxml2 is located on your system. please make sure `pkg-config` is installed.
```

`brew link --force libxml2` で解決した。

```
$ brew link --force libxml2
Linking /usr/local/Cellar/libxml2/2.9.1... 16 symlinks created
```

```
$ ls -la /usr/local/lib/libxml2.*
lrwxr-xr-x  1 [user]  admin  43  3  4 10:57 /usr/local/lib/libxml2.2.dylib -> ../Cellar/libxml2/2.9.1/lib/libxml2.2.dylib
lrwxr-xr-x  1 [user]  admin  37  3  4 10:57 /usr/local/lib/libxml2.a -> ../Cellar/libxml2/2.9.1/lib/libxml2.a
lrwxr-xr-x  1 [user]  admin  41  3  4 10:57 /usr/local/lib/libxml2.dylib -> ../Cellar/libxml2/2.9.1/lib/libxml2.dylib
```