Had the absolutely same issue on Windows 7 x64 with Ruby 2.0.0 and DevKit 4.7.
The following steps helped me.
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gem uninstall mysql2
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Download last MySQL connector from http://cdn.mysql.com/Downloads/Connector-C/mysql-connector-c-noinstall-6.0.2-win32.zip
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Extract it to C:\connector-6.0.2
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gem install mysql2 –platform=ruby — ‘–with-mysql-lib=”C:\connector-6.0.2\lib” –with-mysql-include=”C:\connector-6.0.2\include” –with-mysql-dir=”C:\connector-6.0.2″‘
Or even shorter:
gem install mysql2 –platform=ruby — –with-opt-dir=”C:\connector-6.0.2″