* Fix #3588: Make existing overflow code safe

Signed overflow is undefined behaviour in C and compilers can and do optimized on that basis.

* Speed up our overflow impls

Note this is primarily based on performance of compilers which do not
support __builtin_add_overflow (i.e., not Clang >= 3.4 & gcc >= 5),
mostly looking at several gcc 4 releases (used by older, supported,
RHEL releases and Debian 8 "Jessie") and MSVC.

* Use __builtin_XXX_overflow if available

This is much quicker in general, as these all just then read the overflow flag from the status register.