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

$ bundle install

するとmysql2 0.5.2のところでエラー

Fetching mysql2 0.5.2
Installing mysql2 0.5.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/jun/fout/bb/bu/vendor/bundle/gems/mysql2-0.5.2/ext/mysql2
/Users/jun/.rbenv/versions/2.6.3/bin/ruby -I /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0 -r ./siteconf20190617-8478-1oyt6d2.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/jun/.rbenv/versions/2.6.3/bin/$(RUBY_BASE_NAME)
/Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:467:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:552:in `try_link0'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:570:in `try_link'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:782:in `try_func'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:1069:in `block in have_func'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:959:in `block in checking_for'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:361:in `block (2 levels) in postpone'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:331:in `open'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:361:in `block in postpone'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:331:in `open'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:357:in `postpone'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:958:in `checking_for'
    from /Users/jun/.rbenv/versions/2.6.3/lib/ruby/2.6.0/mkmf.rb:1068: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/jun/fout/bb/bu/vendor/bundle/extensions/x86_64-darwin-18/2.6.0-static/mysql2-0.5.2/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/jun/fout/bb/bu/vendor/bundle/gems/mysql2-0.5.2 for inspection.
Results logged to /Users/jun/fout/bb/bu/vendor/bundle/extensions/x86_64-darwin-18/2.6.0-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.

In Gemfile:
  mysql2

解決策

rubyのバージョンをプロジェクトに合わせる

$ rbenv local {Rubyバージョン}

Gemfile.lockファイル削除

$ rm  -rf Gemfile.lock

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

$ bundle install