[CM] Failed to build S7 Scheme using MSVC when HAVE_OVERFLOW_CHECKS is enabled

[bil at ccrma.Stanford.EDU]

To implement those functions I need the equivalent
of gcc's __builtin_add_overflow and friends.  The
MSVC versions appear to be restricted to unsigned
ints, and only work on the x86 chips.  I'm very
leery of any code Claude or its competitors turns
out.  What do they suggest?