使用rails 3.1.0和win7，gem update mysql2失败了

Question

  

可能重复：
  Ruby gem mysql2 install error

安装了带有rails 3.1.0，mysql 5.1.54和win7的mysql2 0.2.6。需要将其更新为mysql2 0.3.7。但是gem更新mysql2生成的错误如下。将libmysql.dll复制到ruby / bin并没有帮助。

$ gem update mysql2
Updating installed gems
Updating mysql2
Temporarily enhancing PATH to include DevKit...
Building native extensions.  This could take a while...
ERROR:  Error installing mysql2:
        ERROR: Failed to build gem native extension.

        c:/Ruby192/bin/ruby.exe extconf.rb
checking for rb_thread_blocking_region()... yes
checking for rb_wait_for_single_fd()... no
checking for main() in -llibmysql... no
*** 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

有什么想法？感谢。