I play around with a number of Drupal locally and today I finally caved in and decided to add GD support to OS X's webserver. Although I've heard great things about the Entropy PHP packages, I did not want to replace OS X's integrated php installation so my remaining option was to grab a copy of PHP 5.2.8 (the version bundled with Leopard) and compile the module myself. I know there must be quite a few other people who are looking do to the same thing, so I here's a step-by-step guide on how I did it:

(Please note that I assume you have Xcode installed, and that all downloads are saved in the "Downloads" folder in your home. If this is not the case, please move the downloads there first before executing any of the listed commands)

1. Visit the libpng and libjpeg homepages and download the latest release available for both. As of writing, the most recent releases are libjpeg 7 and libpng 1.2.37 - the commands below will need to be adjusted slightly if you are compiling a more recent version.
2. Download PHP 5.2.8, available here
3. Extract PHP and prepare to compile libpng and libjpeg statically:
   
   cd ~/Downloads
tar xfj php-5.2.8.tar.bz2
cd php-5.2.8/ext/gd
mv ~/Downloads/libpng-1.2.37.tar.gz ~/Downloads/jpegsrc.v7.tar.gz .
4. Compile libpng without shared libraries:
   
   tar xfz libpng-1.2.37.tar.gz && cd libpng-1.2.37
./configure --disable-shared --enable-static
make && make install DESTDIR=`pwd`/localinstall
cd ../
5. Compile libjpeg without shared libraries:
   
   tar xfz jpegsrc.v7.tar.gz && cd jpeg-7
cp /usr/share/libtool/config.* .
./configure --disable-shared --enable-static
make && make install DESTDIR=`pwd`/localinstall
cd ../
6. Compile the GD extension and point to our newly compiled libpng and libjpeg static libraries:
   
   cd ~/Downloads/php-5.2.8/ext/gd
phpize
MACOSX_DEPLOYMENT_TARGET=10.5 \
CFLAGS="-O3 -fno-common -arch i686" \
LDFLAGS="-O3 -arch i686" \
CXXFLAGS="-O3 -fno-common -arch i686" \
./configure --with-zlib-dir=/usr \
--with-png-dir=`pwd`/libpng-1.2.37/localinstall/usr/local \
--with-jpeg-dir=`pwd`/jpeg-7/localinstall/usr/local \
--with-freetype-dir=/usr/X11R6 \
--with-xpm-dir=/usr/X11R6
make
sudo make install
7. Copy /private/etc/php.ini.default to /private/etc/php.ini
   • Change the default 'extension_dir' (./) to /usr/lib/php/extensions/no-debug-non-zts-20060613/
   • Add extension=gd.so a bit below extension_dir to enable the gd extension
8. sudo nano /System/Library/LaunchDaemons/org.apache.httpd.plist
   Change the part betweenandso that it looks like this:
   
/usr/bin/arch
-i386
       /usr/sbin/httpd
       -D
       FOREGROUND


   This will ensure that Apache starts in 32 bit mode, which is needed as the gd extension was compiled for i686.

That's all! After a reboot (or toggle Web sharing in the System Preferences), everything should be working. phpinfo() should now confirm that GD library is indeed loaded. If it isn't or if apache refuses to start, try opening Console to check /var/log/apache2/error_log for some useful pointers as to what's wrong.

Helpful source I used for writing this tutorial: HOWTO: Install Habari on Mac OS X Leopard, From Scratch