Building a 64-bit RETRO system:

- change CELL to int64_t in these files:

    source/interfaces/retro-unix.c
    tools/muri.c
    tools/embedimage.c
    tools/extend.c

- update the `n:MIN` and `n:MAX` in source/retro.forth to:

    :n:MAX        (-n)    #9223372036854775806 ;
    :n:MIN        (-n)    #-9223372036854775807 ;

Rebuild and enjoy your new 64-bit RETRO system.