mysql 2のgem を導入したいのに...

#課題
とある課題で。mysqlのgemをインストールしないといけないのですが、以下のエラーにハマって
解決できません。
パスが関係しているようなのですが、よく理解できないままです...

% gem install mysql2 -v '0.4.6'

Fetching: mysql2-0.4.6.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing mysql2:
        ERROR: Failed to build gem native extension.

    current directory: /Users/ts/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mysql2-0.4.6/ext/mysql2
/Users/ts/.rbenv/versions/2.3.3/bin/ruby -r ./siteconf20200406-97934-n7ifg7.rb extconf.rb
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for ruby/thread.h... yes
checking for rb_thread_call_without_gvl() in ruby/thread.h... yes
checking for rb_thread_blocking_region()... no
checking for rb_wait_for_single_fd()... yes
checking for rb_hash_dup()... yes
checking for rb_intern3()... yes
checking for rb_big_cmp()... yes
-----
Using mysql_config at /usr/local/bin/mysql_config
-----
checking for mysql.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 errmsg.h... yes
checking for mysqld_error.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/Cellar/mysql/8.0.19/lib
-----
creating Makefile

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

  /Users/ts/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-19/2.3.0-static/mysql2-0.4.6/mkmf.log

current directory: /Users/ts/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mysql2-0.4.6/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/ts/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mysql2-0.4.6/ext/mysql2
make "DESTDIR="
compiling infile.c
compiling client.c
client.c:869:10: error: use of undeclared identifier 'MYSQL_SECURE_AUTH'; did you mean 'MYSQL_DEFAULT_AUTH'?
    case MYSQL_SECURE_AUTH:
         ^~~~~~~~~~~~~~~~~
         MYSQL_DEFAULT_AUTH
/usr/local/Cellar/mysql/8.0.19/include/mysql/mysql.h:187:3: note: 'MYSQL_DEFAULT_AUTH' declared here
  MYSQL_DEFAULT_AUTH,
  ^
client.c:1291:38: error: use of undeclared identifier 'MYSQL_SECURE_AUTH'; did you mean 'MYSQL_DEFAULT_AUTH'?
  return _mysql_client_options(self, MYSQL_SECURE_AUTH, value);
                                     ^~~~~~~~~~~~~~~~~
                                     MYSQL_DEFAULT_AUTH
/usr/local/Cellar/mysql/8.0.19/include/mysql/mysql.h:187:3: note: 'MYSQL_DEFAULT_AUTH' declared here
  MYSQL_DEFAULT_AUTH,
  ^
2 errors generated.
make: *** [client.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/ts/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/mysql2-0.4.6 for inspection.
Results logged to /Users/ts/.rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-19/2.3.0-static/mysql2-0.4.6/gem_make.out

#やったこと
pathが関係していると思い、ここを参考にして
.zshenvに

PATH=/usr/local/mysql/bin:$PATH
export PATH

を追記しましたがうまくいかず。。。

#もしかしたら
gemのバージョンが関係しているのでしょうか？
gemfileはこんな感じです

gemfil
1source 'https://rubygems.org'
2
3git_source(:github) do |repo_name|
4  repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
5  "https://github.com/#{repo_name}.git"
6end
7
8# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
9gem 'rails', '~> 5.1.1'
10# Use mysql as the database for Active Record
11gem 'mysql2', '>= 0.3.18', '< 0.5'
12# Use Puma as the app server
13gem 'puma', '~> 3.7'
14# Use SCSS for stylesheets
15gem 'sass-rails', '~> 5.0'
16# Use Uglifier as compressor for JavaScript assets
17gem 'uglifier', '>= 1.3.0'
18# See https://github.com/rails/execjs#readme for more supported runtimes
19gem 'therubyracer', platforms: :ruby
20
21# Use CoffeeScript for .coffee assets and views
22gem 'coffee-rails', '~> 4.2'
23# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
24gem 'turbolinks', '~> 5'
25# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
26gem 'jbuilder', '~> 2.5'
27# Use Redis adapter to run Action Cable in production
28# gem 'redis', '~> 3.0'
29# Use ActiveModel has_secure_password
30# gem 'bcrypt', '~> 3.1.7'
31
32# Use Capistrano for deployment
33# gem 'capistrano-rails', group: :development
34
35gem 'config'
36gem 'devise', '~> 4.3'
37gem 'devise-i18n'
38gem 'rails-i18n'
39gem 'ridgepole'
40gem 'bootstrap-sass'
41gem 'jquery-rails'
42gem 'jquery-ui-rails'
43gem 'kaminari'
44gem 'jp_prefecture'
45gem 'paranoia'
46gem 'ransack', github: 'activerecord-hackery/ransack'
47gem 'enum_help'
48gem 'data-confirm-modal'
49gem 'rails-i18n'
50gem 'gmo'
51gem 'mechanize'
52gem 'activerecord-import'
53gem 'aws-sdk-rails'
54
55gem 'whenever', :require => false
56gem 'pry-rails'
57
58# deliver_later
59gem 'delayed_job'
60gem 'delayed_job_active_record'
61
62gem 'daemons'
63
64# for cohort
65gem 'chart-js-rails'
66gem 'gon'
67gem 'slim-rails'
68
69group :development, :test do
70  gem 'rspec-rails', '~> 3.5'
71  gem 'factory_girl_rails'
72  gem 'faker'
73  gem 'json_spec'
74end
75
76group :test do
77  gem 'webmock'
78  gem 'vcr'
79  gem 'database_cleaner'
80  gem 'timecop'
81  gem 'spring-commands-rspec'
82end
83
84group :development do
85  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
86  gem 'web-console', '>= 3.3.0'
87  gem 'listen', '>= 3.0.5', '< 3.2'
88  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
89  gem 'spring'
90  gem 'spring-watcher-listen', '~> 2.0.0'
91  gem 'letter_opener_web'
92  gem 'better_errors'
93  gem 'binding_of_caller'
94end
95
96# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
97gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
98