【Tips】Ubuntuへのmysql2 gemインストール時に発生しうるエラーへの対処

# 問題
Ruby on Rails 等で利用する Ruby の MySQL 用ライブラリ`mysql2`インストールが失敗する。

## 入力
```sh
gem install mysql2
```

## 出力
```sh
Building native extensions. This could take a while...
ERROR:  Error installing mysql2:
        ERROR: Failed to build gem native extension.

    current directory: /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/mysql2-0.5.3/ext/mysql2
/home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/bin/ruby -I /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/2.7.0 -r ./siteconf20200222-2823-u3a2hd.rb extconf.rb
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for rb_wait_for_single_fd()... yes
*** 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=/home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/bin/$(RUBY_BASE_NAME)
        --with-mysql-dir
        --without-mysql-dir
        --with-mysql-include
        --without-mysql-include=${mysql-dir}/include
        --with-mysql-lib
        --without-mysql-lib=${mysql-dir}/lib
        --with-mysql-config
        --without-mysql-config
        --with-mysqlclient-dir
        --without-mysqlclient-dir
        --with-mysqlclient-include
        --without-mysqlclient-include=${mysqlclient-dir}/include
        --with-mysqlclient-lib
        --without-mysqlclient-lib=${mysqlclient-dir}/lib
        --with-mysqlclientlib
        --without-mysqlclientlib
/home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/2.7.0/mkmf.rb:1050:in `block in find_library': undefined method `split' for nil:NilClass (NoMethodError)
        from /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/2.7.0/mkmf.rb:1050:in `collect'
        from /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/2.7.0/mkmf.rb:1050:in `find_library'
        from extconf.rb:87:in `<main>'

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

  /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/extensions/x86_64-linux/2.7.0/mysql2-0.5.3/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/mysql2-0.5.3 for inspection.
Results logged to /home/【ユーザー名】/.anyenv/envs/rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/extensions/x86_64-linux/2.7.0/mysql2-0.5.3/gem_make.out
```

# 環境
- Windows Subsystem for Linux (WSL)
    * Ubuntu 18.04 LTS
- anyenv 1.1.1
    * rbenv 1.1.2-20-g143b2c9
        + ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]

# 原因：MySQLの開発用ライブラリ不足
「Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers.」とあるように、native extensionとして必要なMySQLに関連するライブラリがインストールされていない。

# 解決方法
## 入力
```sh
sudo apt install libmysqld-dev
gem install mysql2
```

## 出力
```
Building native extensions. This could take a while...
Successfully installed mysql2-0.5.3
Parsing documentation for mysql2-0.5.3
Installing ri documentation for mysql2-0.5.3
Done installing documentation for mysql2 after 0 seconds
1 gem installed
```

# 補足
Ubuntu 20.04 LTS で `libmysqld-dev` をインストールしようとする（解決方法の入力1行目にあたる）と以下のメッセージが表示される。

```sh
Package libmysqld-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libmysqlclient-dev libmariadbd-dev libmariadb-dev
```

この場合、メッセージに従い以下の通りに実行すれば良い。

```sh
sudo apt install libmysqlclient-dev
```

# 参考文献
- [Ubuntuでmysql2 gemが入らない。](https://qiita.com/s_osa/items/34d1814b5b213ca29af7)