...but native 64-bit arithmetic isn't supported by my compiler or it isn't enabled. Does any 32-bit compiler on a 32-bit platform have support for 64-bit arithmetic?
I believe that C99 specifies support for the long long int (64 bits or bigger) type as mandatory, so any modern compiler aiming to be C99 compliant should have 64 bits support.
My initial plan was to add a fallback mode able to handle 64 bit arithmetic with 32 bit ints (it shouldn't be too difficult) but until today nobody had asked for it so it just remained undone...
Can you give me the details of your system, OS, compiler, etc? are you sure it doesn't support 64 bit integers in any way?