The root of the problem is that freetype and libpng are installed in non-canonical locations by XCode, in /usr/X11 instead of /usr or /usr/local.
All of the answers already given address the problem by re-building freetype and libpng, either manually or using a package manager like homebrew.
You can, however, get matplotlib to compile by simply symlinking the existing freetype/libpng headers and libraries into the /usr/local tree with:
sudo mkdir -p /usr/local/include
sudo ln -s /usr/X11/include/freetype2/freetype /usr/local/include/freetype
sudo ln -s /usr/X11/include/ft2build.h /usr/local/include/ft2build.h
sudo ln -s /usr/X11/include/png.h /usr/local/include/png.h
sudo ln -s /usr/X11/include/pngconf.h /usr/local/include/pngconf.h
sudo ln -s /usr/X11/include/pnglibconf.h /usr/local/include/pnglibconf.h
sudo mkdir -p /usr/local/lib
sudo ln -s /usr/X11/lib/libfreetype.dylib /usr/local/lib/libfreetype.dylib
sudo ln -s /usr/X11/lib/libpng.dylib /usr/local/lib/libpng.dylib
I prefer to build python packages with pip, so I would then use:
sudo pip install matplotlib
If you don't already have pip, you can install it with easy_install (which comes with OS X):
sudo easy_install pip
I've only tested this on 10.7 (Lion) but I suspect it will also work with 10.6.
It is a little bit of a hack, but I've found it to be the easiest way to get matplotlib installed against the stock python framework that ships with OS X. The stock python framework in 10.7 is actually pretty good, and includes, for instance, a numpy-1.5.1 package that is linked against Apple's optimized BLAS library (Accelerate):
dyldinfo -dylibs /System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/numpy/linalg/lapack_lite.so
for arch x86_64:
attributes dependent dylibs
/System/Library/Frameworks/Accelerate.framework/Versions/A/Accelerate
/usr/lib/libSystem.B.dylib
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