Assuming proper C99 language support, we can select "the int type as big as
a pointer" (most of the time) or "the 64-bit int type" (nanboxing with
REPR_D), and then define everything else automatically.
This simplifies port configuration files. And the types can still be
overridden if needed.
Signed-off-by: Jeff Epler <jepler@unpythonic.net>