INT_ADD_WRAPV and friends - bug fixed

[Paul Eggert]

Eli Zaretskii wrote:
>    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37006

Thanks for reporting that bug. I fixed it by installing the attached into 
Gnulib. While working on the fix I discovered and reported a bug in GCC's 
integer-overflow builtins: see:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91450

The new Gnulib code works around this GCC bug.

As you can see by inspecting the patch, it can be surprisingly difficult to do 
integer overflow checking correctly for all edge cases, particularly given the 
compiler bugs in this area. For example, the Gnulib code simply ignores Clang's 
integer-overflow builtins, because working around Clang's builtins' bugs is more 
trouble than it's worth.

Ad-hoc integer-overflow checking is unfortunately quite error-prone in C-like 
languages, which why INT_ADD_WRAPV etc. are useful.

0001-intprops-support-unsigned-_WRAPV-results.patch
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