From: Christine on
> For a 64 bit binary add the flag "-m64". I haven't done this for mex-files with Matlab, but did need to add this option for other projects. The snippet (from the web) below explains what the various options do.

That was the fix that I got from the GCC list and it did the trick. -arch x86_64 is the architecture option used by Apple's gcc compiler, whereas -m64 is needed for the generic gcc (which I was using). Thank you so much.