Sé que esto se ha pedido varias veces, pero busqué y probé muchas cosas diferentes y nada funcionó.

ERROR:  Error installing mysql2:
    ERROR: Failed to build gem native extension.

        /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/bin/ruby extconf.rb
checking for rb_thread_blocking_region()... yes
checking for mysql.h... yes
checking for errmsg.h... yes
checking for mysqld_error.h... yes
creating Makefile

make
gcc -I. -I/Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/x86_64-darwin10.6.0 -I/Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby/backward -I/Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1 -I. -DHAVE_RB_THREAD_BLOCKING_REGION -DHAVE_MYSQL_H -DHAVE_ERRMSG_H -DHAVE_MYSQLD_ERROR_H -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -I/usr/local/mysql/include  -Os -g -fno-common -fno-strict-aliasing -arch i386 -fno-common -O3 -ggdb -Wextra -Wno-unused-parameter -Wno-parentheses -Wpointer-arith -Wwrite-strings -Wno-missing-field-initializers -Wshorten-64-to-32 -Wno-long-long  -fno-common -pipe -Wall -funroll-loops  -o client.o -c client.c
In file included from /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby.h:32,
                 from ./mysql2_ext.h:4,
                 from client.c:1:
/Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby/ruby.h:108: error: size of array ‘ruby_check_sizeof_long’ is negative
/Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby/ruby.h:112: error: size of array ‘ruby_check_sizeof_voidp’ is negative
In file included from /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby/intern.h:29,
                 from /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby/ruby.h:1327,
                 from /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby.h:32,
                 from ./mysql2_ext.h:4,
                 from client.c:1:
/Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/include/ruby-1.9.1/ruby/st.h:69: error: size of array ‘st_check_for_sizeof_st_index_t’ is negative
make: *** [client.o] Error 1


Gem files will remain installed in /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/gems/1.9.1/gems/mysql2-0.2.6 for inspection.
Results logged to /Users/kishinmanglani/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/gems/1.9.1/gems/mysql2-0.2.6/ext/mysql2/gem_make.out