Instalé el servidor MySQL con el instalador dehttp://dev.mysql.com/downloads/mysql. Luego, como se explica en varias guías, quise instalar la gema mysql con el siguiente comando.

sudo gem install mysql

Esto no funciona y da la siguiente salida.

Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
    ERROR: Failed to build gem native extension.

        /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/bin/ruby extconf.rb --with-mysql-dir=/usr/local/mysql/
checking for mysql_ssl_set()... *** 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
    --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=/Users/luzi/.rvm/rubies/ruby-1.9.3-p429/bin/ruby
    --with-mysql-config
    --without-mysql-config
/Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:461:in `try_link0'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:476:in `try_link'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:619:in `try_func'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:894:in `block in have_func'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:254:in `open'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:254:in `open'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
    from /Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib/ruby/1.9.1/mkmf.rb:893:in `have_func'
    from extconf.rb:45:in `<main>'


Gem files will remain installed in /Users/luzi/.rvm/gems/ruby-1.9.3-p429/gems/mysql-2.9.1 for inspection.
Results logged to /Users/luzi/.rvm/gems/ruby-1.9.3-p429/gems/mysql-2.9.1/ext/mysql_api/gem_make.out

Reinstalé Ruby 1.9.3 varias veces. Las utilidades de línea de comando XCode también están instaladas. También instalé el servidor mysql con homebrew (no traje nada).

También probé algunas opciones para el comando anterior, como--with-mysql-dir=/usr/local/mysql y--with-mysql-config=/usr/local/mysql/bin/mysql_config. Pasando en banderas (sudo env ARCHFLAGS="-arch x86_64" gem install mysql) no ayudó a ninguno.

ACTUALIZAR:

También probé la gema mysql2, con el resultado del mismo error, probablemente porque la gema mysql2 depende de la gema mysql.

ACTUALIZACIÓN 2:

Contenido demkmf.log:

"/opt/local/bin/gcc-apple-4.2 -o conftest -I/Users/luzi/.rvm/rubies/ruby-1.9.3-p429/include/ruby-1.9.1/x86_64-darwin12.4.0 -I/Users/luzi/.rvm/rubies/ruby-1.9.3-p429/include/ruby-1.9.1/ruby/backward -I/Users/luzi/.rvm/rubies/ruby-1.9.3-p429/include/ruby-1.9.1 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -I/opt/local/include -I/usr/local/mysql/include  -Wno-null-conversion -Os -g -fno-strict-aliasing -arch x86_64  -O3 -ggdb -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wshorten-64-to-32 -Wimplicit-function-declaration  -fno-common -pipe conftest.c  -L. -L/Users/luzi/.rvm/rubies/ruby-1.9.3-p429/lib -L/opt/local/lib -L. -L/usr/local/lib -L/opt/local/lib     -L/usr/local/mysql/lib -lmysqlclient  -lruby.1.9.1  -lpthread -ldl -lobjc "
cc1: error: unrecognized command line option "-Wno-null-conversion"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */