What's the gcc version? This is a builtin introduced in GCC7 [https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html](https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html) and Clang 4 (?). Not sure if it has been backported.
We might need a switch for old GCCs.
