installation fails because of activerecord-mysql-adapter missing

[JPT]

* config of redmine fails:

$ apt-get install redmine
...
Setting up redmine (2.3.1-1) ...
dbconfig-common: writing config to /etc/dbconfig-common/redmine/instances/default.conf
Creating config file /etc/redmine/default/database.yml.new with new version
granting access to database redmine_default for redmine@localhost: already exists.
creating database redmine_default: already exists.
dbconfig-common: flushing administrative password
Replacing config file /etc/redmine/default/session.yml with new version
A new secret session key has been generated in /etc/redmine/default/session.yml
Populating database for redmine instance "default".
This may take a while.
rake aborted!
Please install the mysql adapter: `gem install activerecord-mysql-adapter` (cannot load such file -- mysql)
Tasks: TOP => db:migrate => environment
(See full trace by running task with --trace)
Error when running rake db:migrate, check database configuration.
dpkg: error processing redmine (--configure):
 subprocess installed post-installation script returned error exit status 1
...

* installing the mysql adapter fails, too:

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

        /usr/bin/ruby1.9.1 extconf.rb
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lm... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lz... no
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lsocket... no
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lnsl... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lmygcc... no
checking for mysql_query() in -lmysqlclient... no
*** 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 /var/lib/gems/1.9.1/gems/mysql-2.9.1 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/mysql-2.9.1/ext/mysql_api/gem_make.out

* btw, installing mysql2-adapter works:
$gem install activerecord-mysql2-adapter
Fetching: activerecord-mysql2-adapter-0.0.3.gem (100%)
Successfully installed activerecord-mysql2-adapter-0.0.3
1 gem installed
Installing ri documentation for activerecord-mysql2-adapter-0.0.3...
Installing RDoc documentation for activerecord-mysql2-adapter-0.0.3...

* installing libmysqlclient-dev solves the problem with "gem install activerecord-mysql-adapter", but "apt-get install redmine" still fails with the same error message.

Distri: Ubuntu-Server 13.10 armhf
Hardware: Netgear ReadyNAS 104 (ARMv7l cpu)