Recently I found the Python Imaging Library (PIL) would not install properly on my Intel Mac. The error message when trying to install PIL was:

ld: in /Developer/SDKs/MacOSX10.4u.sdk/usr/local/lib/libxml2.2.dylib,
file is not of required architecture for architecture ppc

It turned out that gcc was being called as gcc -arch ppc -arch i386, which is used to build a universal binary on OS X. Since the installed libxml2 was Intel-only, and setuptools did not detect this, the PIL installation failed.

Force gcc to build only an Intel binary when you install PIL, rather than a universal binary, by setting an environment variable:

sudo env ARCHFLAGS="-arch i386" python setup.py install

Since universal binaries are not needed on Intel Macs unless you are preparing them for distribution, this lets you install PIL normally.