pi@raspberrypi:/partage/tom $ gem install 'mysql2'
Building native extensions. This could take a while...
ERROR:  Error installing mysql2:
        ERROR: Failed to build gem native extension.

    current directory: /home/pi/.rvm/gems/ruby-2.6.3/gems/mysql2-0.5.2/ext/mysql2
/home/pi/.rvm/rubies/ruby-2.6.3/bin/ruby -I /home/pi/.rvm/rubies/ruby-2.6.3/lib/ruby/site_ruby/2.6.0 -r ./siteconf20190801-14501-vtb7f.rb extconf.rb
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for rb_wait_for_single_fd()... yes
checking for -lmysqlclient... no
-----
mysql client is missing. You may need to 'apt-get install libmysqlclient-dev' or 'yum install mysql-devel', and try again.
-----
*** 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.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/home/pi/.rvm/rubies/ruby-2.6.3/bin/$(RUBY_BASE_NAME)
        --with-mysql-dir
        --without-mysql-dir
        --with-mysql-include
        --without-mysql-include=${mysql-dir}/include
        --with-mysql-lib
        --without-mysql-lib=${mysql-dir}/lib
        --with-mysql-config
        --without-mysql-config
        --with-mysql-dir
        --without-mysql-dir
        --with-mysql-include
        --without-mysql-include=${mysql-dir}/include
        --with-mysql-lib
        --without-mysql-lib=${mysql-dir}/lib
        --with-mysqlclientlib
        --without-mysqlclientlib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /home/pi/.rvm/gems/ruby-2.6.3/extensions/armv7l-linux/2.6.0/mysql2-0.5.2/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /home/pi/.rvm/gems/ruby-2.6.3/gems/mysql2-0.5.2 for inspection.
Results logged to /home/pi/.rvm/gems/ruby-2.6.3/extensions/armv7l-linux/2.6.0/mysql2-0.5.2/gem_make.out