If you’re a Unix hacker on Mac OS X, chances are you’ll be using a very large number of open-source projects that use GNU autotools: these are the projects that you typically compile with the ./configure script. It turns out that building a Mac OS X “Universal Binary” that runs on both PowerPC and Intel isn’t too hard at all, with the appropriate magic incantations to the ./configure script:

CFLAGS="-isysroot /Developer/SDKs/MacOSX10.4u.sdk -Wl,-syslibroot,/Developer/SDKs/MacOSX10.4u.sdk -arch i386 -arch ppc" \
./configure 

Notes about this:

• You will get a ton of warnings from the linker during the compile that the -syslibroot parameter was ignored since no linking was done. Just ignore them. (If you find out how to shut ld up, do email me!)
• You may need to pass the --disable-dependency-tracking to ./configure, especially for projects that use GNU libtool. Yeah, this means you won’t get proper dependency tracking for your project, so (just like the Universal Binary Programming Guidelines suggests) I’d suggest you compile a universal binary only when you build a proper release package.

Update: Note that this is merely a way to get autotools to build universal binaries. It definitely does not mean that your project will automagically work with on both PowerPC and x86. (In particular, you’ll have to change autoconf-supplied architecture and endian macros such as AC_C_BIGENDIAN: see the autoconf section of the Universal Binary Programming Guidelines for more details.)

Update (2): It seems that this technique has officially been sanctioned by Apple: technical note TN2137 uses the exact CFLAGS that I’ve described here.