RailsでMysqlを使うときに出たエラーの対処

## 概要
以下のようなエラーログがでてきて、mysql2をinstallするのに失敗してしまった。

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

    current directory: /Users/Kento/work/hituji/vendor/bundle/ruby/2.4.0/gems/mysql2-0.4.5/ext/mysql2
/Users/Kento/.rbenv/versions/2.3.0-dev/bin/ruby -r ./siteconf20161220-15918-1p4ufx4.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... no
checking for MYSQL_OPT_SSL_ENFORCE in mysql.h... no
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/5.6.19/lib
-----
creating Makefile

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

  /Users/Kento/work/hituji/vendor/bundle/ruby/2.4.0/extensions/x86_64-darwin-15/2.4.0-static/mysql2-0.4.5/mkmf.log

current directory: /Users/Kento/work/hituji/vendor/bundle/ruby/2.4.0/gems/mysql2-0.4.5/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/Kento/work/hituji/vendor/bundle/ruby/2.4.0/gems/mysql2-0.4.5/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/Kento/work/hituji/vendor/bundle/ruby/2.4.0/gems/mysql2-0.4.5 for inspection.
Results logged to /Users/Kento/work/hituji/vendor/bundle/ruby/2.4.0/extensions/x86_64-darwin-15/2.4.0-static/mysql2-0.4.5/gem_make.out
Using puma 3.6.2
Using sass 3.4.23
Using tilt 2.0.5
Using turbolinks-source 5.0.0
Using tzinfo 1.2.2
Using nokogiri 1.6.8.1
Using rack-test 0.6.3
Using sprockets 3.7.1
Using websocket-driver 0.6.4
Using mime-types 3.1
Using coffee-script 2.4.1
Using uglifier 3.0.4
Using rb-inotify 0.9.7
An error occurred while installing mysql2 (0.4.5), and Bundler cannot continue.
```

## 問題
libsslが見つからないのが原因。

```
ld: library not found for -lssl
```
opensslへのlinkが貼れなくなっているらしいのでlinkを貼るのではなく、pathをしていする。

## 解決策
pathを見てみて、それをbundle.configに入れる

```
$ brew reinstall openssl --force
Warning: Refusing to link: openssl
Linking keg-only openssl means you may end up linking against the insecure,
deprecated system OpenSSL while using the headers from Homebrew's openssl.
Instead, pass the full include/library paths to your compiler e.g.:
  -I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib


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