Rails nokogiriで正常にbundle installできない

Question

bundle installを実行するとnokogiriのlibxml2に関するエラーが発生してしまいます。

Fetching nokogiri 1.8.1
Installing nokogiri 1.8.1 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/macbookpro/Documents/Ruby/members/vendor/bundle/ruby/2.3.0/gems/nokogiri-1.8.1/ext/nokogiri
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20180423-89744-d6nawv.rb extconf.rb --use-system-libraries
--with-iconv-dir=/usr/local/opt/libiconv
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.
pkg-config could not be used to find libxml-2.0
Please install either `pkg-config` or the pkg-config gem per

    gem install pkg-config -v "~> 1.1"

pkg-config could not be used to find libxslt
Please install either `pkg-config` or the pkg-config gem per

    gem install pkg-config -v "~> 1.1"

pkg-config could not be used to find libexslt
Please install either `pkg-config` or the pkg-config gem per

    gem install pkg-config -v "~> 1.1"

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=/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/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/macbookpro/Documents/Ruby/members/vendor/bundle/ruby/2.3.0/extensions/universal-darwin-17/2.3.0/nokogiri-1.8.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/macbookpro/Documents/Ruby/members/vendor/bundle/ruby/2.3.0/gems/nokogiri-1.8.1 for inspection.
Results logged to /Users/macbookpro/Documents/Ruby/members/vendor/bundle/ruby/2.3.0/extensions/universal-darwin-17/2.3.0/nokogiri-1.8.1/gem_make.out

An error occurred while installing nokogiri (1.8.1), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.8.1'` succeeds before bundling.

In Gemfile:
  rails was resolved to 5.0.6, which depends on
    actioncable was resolved to 5.0.6, which depends on
      actionpack was resolved to 5.0.6, which depends on
        actionview was resolved to 5.0.6, which depends on
          rails-dom-testing was resolved to 2.0.3, which depends on
            nokogiri

以下のQiitaを参考にすると
Qiita - OSX への nokogiri 1.6.8.rc3 の install でハマって解決したメモ
libxml2のリンクがされてないことが原因のようですが、実際に見てみると

$ ls -la /usr/local/lib/ | grep libxml2
lrwxr-xr-x   1 macbookpro  admin   43  4 22 02:46 libxml2.2.dylib -> ../Cellar/libxml2/2.9.7/lib/libxml2.2.dylib
lrwxr-xr-x   1 macbookpro  admin   37  4 22 02:46 libxml2.a -> ../Cellar/libxml2/2.9.7/lib/libxml2.a
lrwxr-xr-x   1 macbookpro  admin   41  4 22 02:46 libxml2.dylib -> ../Cellar/libxml2/2.9.7/lib/libxml2.dylib
lrwxr-xr-x   1 macbookpro  admin   39  4 22 02:46 xml2Conf.sh -> ../Cellar/libxml2/2.9.7/lib/xml2Conf.sh

すでにリンクがされています。

エラー文を検索しながらサイトに書いてあることを一通りやってみたのですがうまくいかず...

再度bundle installをやって見ても同じエラーになってしまいます。どうすれば、正常に行われるでしょうか？
解決策をご教授いただけると助かります。

#行ったこと

$ bundle config build.nokogiri --use-system-libraries
You are replacing the current global value of build.nokogiri, which is currently "--use-system-libraries --with-iconv-dir=/usr/local/opt/libiconv"

xcodeのコマンドラインツールのインストール

$ xcode-select --install
xcode-select: error: command line tools are already installed, use "Software Update" to install updates

リンクし直しても当然、すでにされていると言われる。
「brewにいくつかインストールが必要だ」と書いてあっても、下記の環境の通り、全て揃っている。

環境

macOS High Sierra
gemにはすでに以下がインストール済みです。

nokogiri (1.5.6)
pkg-config (1.3.0)

homebrewには

libiconv
libxslt
pkg-config
libxml2

Answer 1

解決をいたしました！

Answer 2

つhttp://www.nokogiri.org/tutorials/installing_nokogiri.html

Comments

[ruby_0ct]

ご回答ありがとうございます。
gemにはpkg-config (1.3.0)とnokogiri (1.5.6)がインストールされています。
homebrewにもpkg-configがインストールされているのですが、パスの問題でしょうか？