前提条件

mac OS
Rails 6.0.3.2
ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-darwin19]

問題点

某オンラインスクールの教材を見ながらローカル環境でプロジェクトを作成していたところ、bundle installで以下のエラーが発生した。

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/ebinashohei/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3/ext/mysql2
/Users/ebinashohei/.rbenv/versions/2.6.3/bin/ruby -I /Users/ebinashohei/.rbenv/versions/2.6.3/lib/ruby/2.6.0 -r ./siteconf20200622-74325-qvfli8.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_1/lib
-----
creating Makefile

current directory: /Users/ebinashohei/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/ebinashohei/.rbenv/versions/2.6.3/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/ebinashohei/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/mysql2-0.5.3 for inspection.
Results logged to /Users/ebinashohei/.rbenv/versions/2.6.3/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

解決方法

同じようなエラーを解決してくれていたため以下に示す。

$bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib"
$ bundle install

参考サイト
https://qiita.com/SAYJOY/items/dd7c8fc7a3647e7ff969