Amazon Linuxにmysql2を入れようとしたらエラーがでる

急いでいる人のために、先に解決方法を書いておくと
$ sudo yum install mysql-devel
をしてから
$ gem install mysql2
をすれば良い。

---

EC2でAmazon Linuxを借りて、
gem install mysql2
をしたら、以下の様に怒られたので対処法を書いておく。

Building native extensions.  This could take a while...
ERROR:  Error installing mysql2:
    ERROR: Failed to build gem native extension.

    /home/ec2-user/.rbenv/versions/2.1.2/bin/ruby 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()... yes
checking for rb_wait_for_single_fd()... yes
checking for rb_hash_dup()... yes
checking for rb_intern3()... yes
-----
Using mysql_config at /usr/bin/mysql_config
-----
checking for mysql.h... no
checking for mysql/mysql.h... no
-----
mysql.h is missing.  please check your installation of mysql and try again.
-----
*** 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=/home/ec2-user/.rbenv/versions/2.1.2/bin/ruby
    --with-mysql-dir
    --without-mysql-dir
    --with-mysql-include
    --without-mysql-include=${mysql-dir}/include
    --with-mysql-lib
    --without-mysql-lib=${mysql-dir}/lib
    --with-mysql-config
    --without-mysql-config

extconf failed, exit code 1

Gem files will remain installed in /home/ec2-user/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mysql2-0.3.16 for inspection.
Results logged to /home/ec2-user/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/extensions/x86_64-linux/2.1.0-static/mysql2-0.3.16/gem_make.out

原因はmysql.hがないことらしいので、
$ sudo yum install mysql-devel
とすれば良かった。

その後に、もう一度
$ gem install mysql2
をすれば成功する。