GCC 7 __builtin_add_overflow supports a new usage form, where the
last argument is a null pointer, and which merely returns 1 if an
overflow would occur. This is a constant expression if all
arguments are constants, and should generate faster code when code
needs to be generated.
* lib/intprops.h (_GL_HAS_BUILTIN_OVERFLOW_WITH_NULL): New macro.
(_GL_ADD_OVERFLOW, _GL_SUBTRACT_OVERFLOW, _GL_MULTIPLY_OVERFLOW):
Use builtin operations if available.
(INT_NEGATE_OVERFLOW): Prefer INT_SUBTRACT_OVERFLOW if builtin
operations are available, as it's almost surely faster.