gem install libxml-ruby -v '3.1.0' に失敗します

Building native extensions. This could take a while...
ERROR:  Error installing libxml-ruby:
    ERROR: Failed to build gem native extension.
    current directory: /Users/shigerunakajima/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/libxml-ruby-3.1.0/ext/libxml
/Users/shigerunakajima/.rbenv/versions/2.5.5/bin/ruby -r ./siteconf20200414-17043-wpqllw.rb extconf.rb
/Users/shigerunakajima/.rbenv/versions/2.5.5/bin/ruby: warning: shebang line ending with \r may cause problems
checking for libxml/xmlversion.h in /opt/include/libxml2,/opt/local/include/libxml2,/usr/local/include/libxml2,/usr/include/libxml2... no
*** 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/shigerunakajima/.rbenv/versions/2.5.5/bin/$(RUBY_BASE_NAME)
    --with-xml2-config
    --without-xml2-config
    --with-xml2-dir
    --without-xml2-dir
    --with-xml2-include
    --without-xml2-include=${xml2-dir}/include
    --with-xml2-lib
    --without-xml2-lib=${xml2-dir}/lib
 extconf failure: need libxml2.
    Install the library or try one of the following options to extconf.rb:
      --with-xml2-config=/path/to/xml2-config
      --with-xml2-dir=/path/to/libxml2
      --with-xml2-lib=/path/to/libxml2/lib
      --with-xml2-include=/path/to/libxml2/include
To see why this extension failed to compile, please check the mkmf.log which can be found here:
  /Users/shigerunakajima/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/extensions/x86_64-darwin-18/2.5.0/libxml-ruby-3.1.0/mkmf.log
extconf failed, exit code 1
Gem files will remain installed in /Users/shigerunakajima/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/libxml-ruby-3.1.0 for inspection.
Results logged to /Users/shigerunakajima/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/extensions/x86_64-darwin-18/2.5.0/libxml-ruby-3.1.0/gem_make.out

libxml2が見つからないみたいです。

~ brew install libxml2
Warning: libxml2 2.9.10_1 is already installed and up-to-date
To reinstall 2.9.10_1, run `brew reinstall libxml2`

Homebrew的にはインストール済みです。

checking for libxml/xmlversion.h in /opt/include/libxml2,/opt/local/include/libxml2,/usr/local/include/libxml2,/usr/include/libxml2... no

とエラーメッセージが出ています。ヘッダファイル自体はあります。

~ ls /usr/local/opt/libxml2/include/libxml2/libxml/xmlversion.h
/usr/local/opt/libxml2/include/libxml2/libxml/xmlversion.h

ヘッダファイルを探すパスに /usr/local/opt/libxml2/include/libxml2 が入っていないのが問題のようです。

~ bundle config build.libxml-ruby --with-xml2-config=/usr/local/opt/libxml2/bin/xml2-config
~ bundle config
Settings are listed in order of priority. The top value will be used.
build.libxml-ruby
Set for the current user (/Users/shigerunakajima/.bundle/config): "--with-xml2-config=/usr/local/opt/libxml2/bin/xml2-config"

gem installは失敗します。だってbundle configだもん。 でもbundle installは成功するので、大丈夫でした！