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Installing the native MySQL-Driver for Ruby on my MacBook Pro (Mac OS X 10.4.8) resulted in the following error after running
sudo gem install mysql -- --with-mysql-dir=/usr/local/mysql.

[...]
Building native extensions.  This could take a while...
mysql.c: In function Init_mysql:
mysql.c:2015: error: ulong undeclared (first use in this function)
mysql.c:2015: error: (Each undeclared identifier is reported only once
mysql.c:2015: error: for each function it appears in.)

It seems that ulong hasn't been declared in mysql.c You've got to add #define ulong unsigned long to mysql.c before you can compile the sources on a intel based mac.

I created a small patch file which you can use if you like (i assume that you have MySQL installed in /usr/local/mysql and that your Ruby Gem INSTALLATION DIRECTORY installation is /usr/local/lib/ruby/gems/1.8 - you can check this running gem environment).

# fetch patch file
cd /tmp; 
curl -O http://download.juretta.com/ruby/mac-install/\
intel-mac-mysql-gem.patch

# Assuming that Ruby Gems is located in /usr/local ...
cd /usr/local/lib/ruby/gems/1.8/gems/mysql-2.7

# Makefile
sudo ruby extconf.rb install mysql -- \
--with-mysql-dir=/usr/local/mysql/

# Backup
sudo cp mysql.c mysql.c-orig

# Patch mysql.c
sudo patch mysql.c /tmp/intel-mac-mysql-gem.patch

# Make
sudo make
# Install
sudo make install

For your convenience there is shell script that builds the native mysql gem including the patch.
Just run

macbook:~ stefan$ curl http://download.juretta.com/ruby/mac-install/\
fix-mysql-gem-intel-mac.sh | /bin/bash

You must provide your superuser password. !Please look at http://download.juretta.com/ruby/mac-install/fix-mysql-gem-intel-mac.sh before you run the script! It might do evil... (it won't though)