I agree with @carlosmn here, we shouldn't be trying to force anything.  Can we be fancy on Mac and build fat binaries?

    if [ `uname -s` = "Darwin" ]; then
        CFLAGS=-arch x86 -arch amd64
        LDFLAGS=-bundle
    fi

If you want to be really crazy you can add `-arch ppc` on Mac OS < 10.8 and `-arch ppc64` on Mac OS 10.5 only.  But that would be really crazy.