SDLMAME will auto detect your CPU type (PPC or Intel), and your system type (32 bit or 64 bit), and build accordingly.
actually, on my macbook pro with Snow Leopard, MAME builds 32-bit even if the OS supports 64-bit apps. hence, I had to add PTR64=1. is this expected? or is the identification process at fault (and if this is the case, can I help to fix it?)