gem install mysql2エラー（Error installing mysql2）

mysql２をインストールした際エラーが出たため、情報共有します。

インストールコマンド

```
gem install mysql2
```
エラー内容

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

    current directory: /Users/aki/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/mysql2-0.5.3/ext/mysql2
/Users/aki/.rbenv/versions/2.7.0/bin/ruby -I /Users/aki/.rbenv/versions/2.7.0/lib/ruby/2.7.0 -r ./siteconf20200324-90394-m50btx.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/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... no
-----
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/Cellar/mysql/8.0.19/lib
-----
creating Makefile

current directory: /Users/aki/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/aki/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR="
compiling client.c
client.c:787:14: warning: incompatible pointer types passing 'VALUE (void *)' (aka 'unsigned long (void *)') to parameter of type 'VALUE (*)(VALUE)' (aka 'unsigned long (*)(unsigned long)') [-Wincompatible-pointer-types]
  rb_rescue2(do_send_query, (VALUE)&args, disconnect_and_raise, self, rb_eException, (VALUE)0);
             ^~~~~~~~~~~~~
/Users/aki/.rbenv/versions/2.7.0/include/ruby-2.7.0/ruby/ruby.h:1988:25: note: passing argument to parameter here
VALUE rb_rescue2(VALUE(*)(VALUE),VALUE,VALUE(*)(VALUE,VALUE),VALUE,...);
                        ^
client.c:795:16: warning: incompatible pointer types passing 'VALUE (void *)' (aka 'unsigned long (void *)') to parameter of type 'VALUE (*)(VALUE)' (aka 'unsigned long (*)(unsigned long)') [-Wincompatible-pointer-types]
    rb_rescue2(do_query, (VALUE)&async_args, disconnect_and_raise, self, rb_eException, (VALUE)0);
               ^~~~~~~~
/Users/aki/.rbenv/versions/2.7.0/include/ruby-2.7.0/ruby/ruby.h:1988:25: note: passing argument to parameter here
VALUE rb_rescue2(VALUE(*)(VALUE),VALUE,VALUE(*)(VALUE,VALUE),VALUE,...);
                        ^
2 warnings generated.
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/aki/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/mysql2-0.5.3 for inspection.
Results logged to /Users/aki/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/extensions/x86_64-darwin-19/2.7.0/mysql2-0.5.3/gem_make.out
```
## 原因
どうやらsslが見つからないとのこと。

```
library not found for -lssl
```

とりあえずインストールはされていたため、アップグレード。


```
brew upgrade openssl
```
すでにインストールされていた。

```
Updating Homebrew...
Warning: openssl 1.1.1d already installed
```
gem installにオプションを付けインストールしてみることにしたら実行できた！

```
gem install mysql2 -v '0.5.2' -- --with-ldflags=-L/usr/local/opt/openssl/lib --with-cppflags=-I/usr/local/opt/openssl/include
```

##まとめ
SSLのエラーがgithubでも起こったため、一度集中的に調べて記事にしておこうと思います。

##参考

もはやこのページの通り実行しただけです笑
[githubページ](https://gist.github.com/fernandoaleman/ee3ac6957c2ba4f7d7d33a251d58b191)