title: Problem installing gems
body: <p>I had recently used datamapper, and then all of a sudden it didn't work.  I keep getting this error:</p>
  
  <pre><code>/Users/paul/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require': no such file to load -- dm-mysql-adapter (LoadError)
      from /Users/paul/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
      from /Users/paul/.rvm/gems/ruby-1.9.2-p180/gems/dm-core-1.1.0/lib/dm-core/adapters.rb:163:in `load_adapter'
      from /Users/paul/.rvm/gems/ruby-1.9.2-p180/gems/dm-core-1.1.0/lib/dm-core/adapters.rb:133:in `adapter_class'
      from /Users/paul/.rvm/gems/ruby-1.9.2-p180/gems/dm-core-1.1.0/lib/dm-core/adapters.rb:13:in `new'
      from /Users/paul/.rvm/gems/ruby-1.9.2-p180/gems/dm-core-1.1.0/lib/dm-core.rb:219:in `setup'
  </code></pre>
  
  <p>Then, when I try to load the dm-mysql-adapter, it gives me this error:</p>
  
  <pre><code>/Users/paul/.rvm/rubies/ruby-1.9.2-p180/bin/ruby extconf.rb --with-mysql-config=/usr/bin/mysql_config
  *** 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.
  </code></pre>
  
  <p>I've seen similar error messages when I'm googling, but none of the responses seem to work for me...</p>