我在 Digital Ocean 云服务器上安装 Redmine 时遇到了这个问题（我使用的是 Centos 7）。在 I 之后bundle install，我收到此错误消息，
An error occurred while installing mysql2 (0.3.16), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.3.16'` succeeds before bundling.
然后我尝试使用这个命令安装 mysql2
gem install mysql2
弹出一条错误消息，
Building native extensions. This could take a while...
ERROR: Error installing mysql2:
ERROR: Failed to build gem native extension.
/usr/bin/ruby -r ./siteconf20140913-16462-tdplhf-0.rb extconf.rb
checking for ruby/thread.h... no
checking for rb_thread_blocking_region()... no
checking for rb_wait_for_single_fd()... no
checking for rb_hash_dup()... no
checking for rb_intern3()... no
-----
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=/usr/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 /usr/lib64/ruby/gems/1.8/gems/mysql2-0.3.16 for inspection.
Results logged to /usr/lib64/ruby/gems/1.8/extensions/x86_64-linux/1.8/mysql2-0.3.16/gem_make.out
我按照本教程如何设置 Ruby on Rails：https ://www.digitalocean.com/community/tutorials/how-to-install-ruby-on-rails-on-centos-6
和Redmine：http://www.redmine.org/projects/redmine/wiki/How_to_Install_Redmine_on_CentOS_（详细）