From ./configure --help:
--with-gd[=DIR] Include GD support where DIR is GD install prefix.
If DIR is not set, the bundled GD library will be used.
So, it's --with-gd. That said, I don't believe PHP 4.3 works correctly on MacOS X with the bundled GD lib. This change landed very late in the release process and I don't think all the MacOS-specific fixes were made.
Try it and let me know. If it works, great! If it doesn't, don't be surprised. I'll continue to fiddle with this and let you know when I get this working.