That's Zsnes, actually... Snes9x, iirc, is fairly portable as these things go. I'm pretty certain its 100% C/C++, and it *may* have an SDL version, although of that, I'm not so sure...
Zsnes on the other hand, is pretty much entirely x86 assembler, and therefore not worth touching with a bargepole, unless you're really a fan of X86 assembler and the conversion of it to ARM asm
EDIT: That is, idd, the PC version of Snes9x I'm talking about... OS9xGP obviously does include a large ammount of assembler. Assuming, however, that its possible to get this to compile in the first place, that shouldn't be a problems seeing as it is ARM ASM, not X86 ASM, and therefore will quite happily execute on the GP2x