On my fresh Ubuntu 11.04 box running Ruby 1.9.2 instead of the standard Ruby 1.8, I ran into some undocumented errors while installing the mysql2 gem. Here’s what I was seeing:

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

        /usr/bin/ruby1.9.1 extconf.rb
          <internal:lib/rubygems/custom_require>:29:in `require':
          no such file to load -- mkmf (LoadError)
  from <internal:lib/rubygems/custom_require>:29:in `require'
  from extconf.rb:2:in `<main>'

Gem files will remain installed in
  /usr/lib/ruby/gems/1.9.1/gems/mysql2-0.3.2 for inspection.
Results logged to
  /usr/lib/ruby/gems/1.9.1/gems/mysql2-0.3.2/ext/mysql2/gem_make.out

This obviously isn’t all that helpful, nor did I find anything all that useful on Google – thankfully the issue is easy to solve. All that’s needed is to ensure that the following packages are installed:

sudo apt-get install libmysqlclient-dev libmysql-ruby1.9 ruby1.9.1-dev

Then you can try installing the gem, it should work, hopefully.