bundle install 時、mysql2でエラー

#実行環境
```terminal
$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.14.2
$ ruby -v
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]
$ rails -v
Rails 5.1.2
```

#エラー内容

gemfileに`gem 'mysql2'`を記述し`bundle install`を行うと下記のようなエラーが発生。

```terminal

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

    current directory: /Users/{username}/rails_app/test-mysql/vendor/bundle/gems/mysql2-0.5.2/ext/mysql2
/Users/{username}/.rbenv/versions/2.5.1/bin/ruby -I /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/site_ruby/2.5.0 -r
./siteconf20181226-54895-ler7vx.rb extconf.rb --with-ldflags\=-L/usr/local/opt/openssl/lib\
--with-cppflags\=-I/usr/local/opt/openssl/include
checking for rb_absint_size()... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
  --with-opt-dir
  --without-opt-dir
  --with-opt-include
  --without-opt-include=${opt-dir}/include
  --with-opt-lib
  --without-opt-lib=${opt-dir}/lib
  --with-make-prog
  --without-make-prog
  --srcdir=.
  --curdir
  --ruby=/Users/{username}/.rbenv/versions/2.5.1/bin/$(RUBY_BASE_NAME)
/Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file.
(RuntimeError)
You have to install development tools first.
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:541:in `try_link0'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:559:in `try_link'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:770:in `try_func'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:1057:in `block in have_func'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:947:in `block in checking_for'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:350:in `block (2 levels) in postpone'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:320:in `open'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:350:in `block in postpone'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:320:in `open'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:346:in `postpone'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:946:in `checking_for'
  from /Users/{username}/.rbenv/versions/2.5.1/lib/ruby/2.5.0/mkmf.rb:1056:in `have_func'
  from extconf.rb:25:in `<main>'

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

  /Users/{username}/rails_app/test-mysql/vendor/bundle/extensions/x86_64-darwin-17/2.5.0-static/mysql2-0.5.2/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/{username}/rails_app/test-mysql/vendor/bundle/gems/mysql2-0.5.2 for inspection.
Results logged to
/Users/{username}/rails_app/test-mysql/vendor/bundle/extensions/x86_64-darwin-17/2.5.0-static/mysql2-0.5.2/gem_make.out

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.
```

#意味がなかったこと(直接解決には繋がらなかったこと)
```terminal
$ xcode-select --install
```
=> すでに入っとるわ

```terminal
$ bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib --with-cppflags=-I/usr/local/opt/openssl/include"
```
=>エラー、そして`/vendor/bundle/extensions/x86_64-darwin-17/2.5.0-static/mysql2-0.5.2/mkmf.log`に下記の記述。

```
clang: error: unsupported option '--with-cppflags=-I/usr/local/opt/openssl/include'
```

#解決策
```terminal
$ bundle config --local build.mysql2 "--with-cppflags=-I/usr/local/opt/openssl/include"
```
または

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

の後に

```terminal
$ bundle install
```
```
Bundle complete! 19 Gemfile dependencies, 80 gems now installed.
Bundled gems are installed into `./vendor/bundle`
```

`"--with-ldflags=-L/usr/local/opt/openssl/lib --with-cppflags=-I/usr/local/opt/openssl/include"`と、オプションを2つつけると通らないみたいですね。

長い道のりであった...