Just a quick note on a problem I ran into this morning.

I needed the mysql gem so I ran gem install mysql only to find I got the following error:

*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.

The solution was to use the following:
gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config

It should work as long as the path to mysql_config is correct, you may need to change the path for your own system. You can check the path to mysql_config with: which mysql_config