環境 && 起こったこと
OS: macOS Sierra
Ruby: 2.3.1
Rails: 5.0.0.1
bundle install したときに mysql2 のところでこけた。
$ bundle install
(諸々のgemのインストールのログは省略）
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
current directory: /Users/akito/git/media/vendor/bundle/gems/mysql2-0.4.4/ext/mysql2
/Users/akito/.rbenv/versions/2.3.1/bin/ruby -r ./siteconf20160929-55293-192vx35.rb extconf.rb
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
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Using mysql_config at /usr/local/bin/mysql_config
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checking for mysql.h... yes
checking for errmsg.h... yes
checking for mysqld_error.h... yes
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Don't know how to set rpath on your system, if MySQL libraries are not in path mysql2 may not load
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Setting libpath to /usr/local/Cellar/mysql/5.7.14/lib
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creating Makefile
To see why this extension failed to compile, please check the mkmf.log which can be found here:
/Users/akito/git/media/vendor/bundle/extensions/x86_64-darwin-15/2.3.0-static/mysql2-0.4.4/mkmf.log
current directory: /Users/akito/git/media/vendor/bundle/gems/mysql2-0.4.4/ext/mysql2
make "DESTDIR=" clean
current directory: /Users/akito/git/media/vendor/bundle/gems/mysql2-0.4.4/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/akito/git/media/vendor/bundle/gems/mysql2-0.4.4 for inspection.
Results logged to
/Users/akito/git/media/vendor/bundle/extensions/x86_64-darwin-15/2.3.0-static/mysql2-0.4.4/gem_make.out
An error occurred while installing mysql2 (0.4.4), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.4.4'` succeeds before bundling.
capybara-webkitとかではこんな感じのことが稀によく起こるのでなんかもういいやってなってるけど、mysql2は初めて見た。
ログの中でld: library not found for -lsslとか言われたけど別にOpenSSL入ってるしなーとか思いながらググってると、こんなのをStackOverflowでみつけた。
Apple has deprecated use of OpenSSL in favor of its own TLS and crypto libraries
Generally there are no consequences of this for you. If you build your own software and it requires this formula, you'll need to add to your build variables:
LDFLAGS: -L/usr/local/opt/openssl/lib
CPPFLAGS: -I/usr/local/opt/openssl/include
PKG_CONFIG_PATH: /usr/local/opt/openssl/lib/pkgconfig
というわけで、（雑な解釈だけど）ビルドのときにLDFLAGSとかCPPFLAGSとかのパスを追加しろって言っているので、回答にもあったコマンドを実行してパスを追加します。
$ bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib --with-cppflags=-I/usr/local/opt/openssl/include"
このあと bundle install したら、無事にインストールされました。