Short note to myself and whom it my concern. If you try to sudo gem install mysql on a Debian system and it responds, after some working, with:
Building native extensions. This could take a while...
ERROR: Error installing mysql:
ERROR: Failed to build gem native extension.
(...)
*** 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.
(...)
Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/mysql-2.7 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/mysql-2.7/gem_make.out
then this should be the solution for you: First you install libmysqlclient15-dev, which generates a mysql_config file, and the install the gem using the config.