【Rails6】MySQL2がbundle installできない

Rails6でアプリケーションをnewしてbundle installすると、下記のエラーが出てbundle installできない。

```
Fetching mysql2 0.5.3
Installing mysql2 0.5.3 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/jun/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3/ext/mysql2
/Users/jun/.rbenv/versions/2.6.5/bin/ruby -I /Users/jun/.rbenv/versions/2.6.5/lib/ruby/2.6.0 -r ./siteconf20200830-5266-esdr5q.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/8.0.21/lib
-----
creating Makefile

current directory: /Users/jun/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/jun/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3/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/jun/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3 for inspection.
Results logged to /Users/jun/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/mysql2-0.5.3/gem_make.out

An error occurred while installing mysql2 (0.5.3), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.5.3' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  mysql2

```

## 解決策

以下のコマンドを実行して

```
$ bundle config --local build.mysql2 "--with-cppflags=-I/usr/local/opt/openssl/include"
$ bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib"
```

bundle installをすると通った

```
$ bundle install
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching gem metadata from https://rubygems.org/............
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Using rake 13.0.1
Using concurrent-ruby 1.1.7
Using i18n 1.8.5
Using minitest 5.14.1
Using thread_safe 0.3.6
Using tzinfo 1.2.7
Using zeitwerk 2.4.0
Using activesupport 6.0.3.2
Using builder 3.2.4
Using erubi 1.9.0
Using mini_portile2 2.4.0
Using nokogiri 1.10.10
Using rails-dom-testing 2.0.3
Using crass 1.0.6
Using loofah 2.7.0
Using rails-html-sanitizer 1.3.0
Using actionview 6.0.3.2
Using rack 2.2.3
Using rack-test 1.1.0
Using actionpack 6.0.3.2
Using nio4r 2.5.2
Using websocket-extensions 0.1.5
Using websocket-driver 0.7.3
Using actioncable 6.0.3.2
Using globalid 0.4.2
Using activejob 6.0.3.2
Using activemodel 6.0.3.2
Using activerecord 6.0.3.2
Using mimemagic 0.3.5
Using marcel 0.3.3
Using activestorage 6.0.3.2
Using mini_mime 1.0.2
Using mail 2.7.1
Using actionmailbox 6.0.3.2
Using actionmailer 6.0.3.2
Using actiontext 6.0.3.2
Using public_suffix 4.0.5
Using addressable 2.7.0
Using bindex 0.8.1
Using msgpack 1.3.3
Using bootsnap 1.4.8
Using bundler 2.1.4
Using byebug 11.1.3
Using regexp_parser 1.7.1
Using xpath 3.2.0
Using capybara 3.33.0
Using childprocess 3.0.0
Using ffi 1.13.1
Using jbuilder 2.10.0
Using rb-fsevent 0.10.4
Using rb-inotify 0.10.1
Using listen 3.2.1
Using method_source 1.0.0
Fetching mysql2 0.5.3
Installing mysql2 0.5.3 with native extensions
Using puma 4.3.5
Fetching rack-proxy 0.6.5
Installing rack-proxy 0.6.5
Using thor 1.0.1
Using railties 6.0.3.2
Using sprockets 4.0.2
Using sprockets-rails 3.2.1
Using rails 6.0.3.2
Using rubyzip 2.3.0
Using sassc 2.4.0
Using tilt 2.0.10
Using sassc-rails 2.1.2
Using sass-rails 6.0.0
Using selenium-webdriver 3.142.7
Using spring 2.1.1
Using spring-watcher-listen 2.0.1
Using turbolinks-source 5.2.0
Using turbolinks 5.2.1
Using web-console 4.0.4
Fetching webdrivers 4.4.1
Installing webdrivers 4.4.1
Fetching webpacker 4.3.0
Installing webpacker 4.3.0
Bundle complete! 17 Gemfile dependencies, 74 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
```

参考
https://qiita.com/fukuda_fu/items/463a39406ce713396403
https://stackoverflow.com/questions/6935146/bundle-update-install-fail-after-os-x-lion-upgrade-rails-3/26203229