mysql2 のインストール時に`ld: library not found for -lssl` が発生した場合の対処 Mac版

## 何これ？

- Ruby on RailsでMySQLをセットしようとしたら、mysql2のgemのインストールに失敗してハマった
- その問題を解決した際のメモ

## 問題

- `Installing mysql2 0.5.2 with native extensions`のインストールで、ERRORが発生
- `gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'`っていうから、試してみたけど、やっぱりエラーが発生した

```bash:rails_new

$ rails new myapp -d mysql
・
（中略）
・
Fetching mysql2 0.5.2
Installing mysql2 0.5.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
・
・
・
An error occurred while installing mysql2 (0.5.2), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  mysql2
```

```bash:gem_install_mysql2
$ gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'
Building native extensions. This could take a while...
ERROR:  Error installing mysql2:
	ERROR: Failed to build gem native extension.

    current directory: /Users/yfujisawa/.rbenv/versions/2.6.0/lib/ruby/gems/2.6.0/gems/mysql2-0.5.2/ext/mysql2
/Users/yfujisawa/.rbenv/versions/2.6.0/bin/ruby -I /Users/yfujisawa/.rbenv/versions/2.6.0/lib/ruby/2.6.0 -r ./siteconf20190131-60046-18fu8vb.rb extconf.rb
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for rb_wait_for_single_fd()... yes
-----
Using mysql_config at /usr/local/opt/mysql@5.7/bin/mysql_config
-----
checking for mysql.h... yes
checking for errmsg.h... yes
checking for SSL_MODE_DISABLED in mysql.h... yes
checking for SSL_MODE_PREFERRED in mysql.h... yes
checking for SSL_MODE_REQUIRED in mysql.h... yes
checking for SSL_MODE_VERIFY_CA in mysql.h... yes
checking for SSL_MODE_VERIFY_IDENTITY in mysql.h... yes
checking for MYSQL.net.vio in mysql.h... yes
checking for MYSQL.net.pvio in mysql.h... no
checking for MYSQL_ENABLE_CLEARTEXT_PLUGIN in mysql.h... yes
checking for SERVER_QUERY_NO_GOOD_INDEX_USED in mysql.h... yes
checking for SERVER_QUERY_NO_INDEX_USED in mysql.h... yes
checking for SERVER_QUERY_WAS_SLOW in mysql.h... yes
checking for MYSQL_OPTION_MULTI_STATEMENTS_ON in mysql.h... yes
checking for MYSQL_OPTION_MULTI_STATEMENTS_OFF in mysql.h... yes
checking for my_bool in mysql.h... yes
-----
Don't know how to set rpath on your system, if MySQL libraries are not in path mysql2 may not load
-----
-----
Setting libpath to /usr/local/opt/mysql@5.7/lib
-----
creating Makefile

current directory: /Users/yfujisawa/.rbenv/versions/2.6.0/lib/ruby/gems/2.6.0/gems/mysql2-0.5.2/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/yfujisawa/.rbenv/versions/2.6.0/lib/ruby/gems/2.6.0/gems/mysql2-0.5.2/ext/mysql2
make "DESTDIR="
compiling client.c
compiling infile.c
compiling mysql2_ext.c
compiling result.c
compiling statement.c
linking shared-object mysql2/mysql2.bundle
ld: library not found for -lssl
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [mysql2.bundle] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/yfujisawa/.rbenv/versions/2.6.0/lib/ruby/gems/2.6.0/gems/mysql2-0.5.2 for inspection.
Results logged to /Users/yfujisawa/.rbenv/versions/2.6.0/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-18/2.6.0-static/mysql2-0.5.2/gem_make.out
training_gym_lesson_worker2 $ bundle config build.mysql2 --with-opt-lib=/usr/local/opt/openssl/lib --with-opt-include=-I/usr/local/opt/openssl/include
Your application has set build.mysql2 to "--with-opt-lib=-L/usr/local/opt/openssl/lib --with-opt-include=-I/usr/local/opt/openssl/include". This will override the global value you are currently setting
```

## 解決

mysql2のIssueが上がっていてのでそれを参考にした
https://github.com/brianmario/mysql2/issues/795

まずは、opensslのインストール

```bash
$ brew install openssl
```

次に、`with-cflags`ってオプションをつけてmysql2をインストール

```
$ gem install mysql2 -- --with-cflags=\"-I/usr/local/opt/openssl/include\" --with-ldflags=\"-L/usr/local/opt/openssl/lib\"
Building native extensions with: '--with-cflags="-I/usr/local/opt/openssl/include" --with-ldflags="-L/usr/local/opt/openssl/lib"'
This could take a while...
Successfully installed mysql2-0.5.2
Parsing documentation for mysql2-0.5.2
Installing ri documentation for mysql2-0.5.2
Done installing documentation for mysql2 after 0 seconds
1 gem installed
```

すると、無事にmysql2がインストールできました！！

## 補足

試していないのですが以下の方法がベストみたいです。

```bash
$ brew install openssl
$ bundle config --global build.mysql2 --with-opt-dir="$(brew --prefix openssl)"
$ bundle install
```