* lib/intprops.h (__has_builtin): Move earlier.
(_GL_HAS_BUILTIN_OVERFLOW): Rename from
_GL_HAS_BUILTIN_OVERFLOW_WITH_NULL and don't worry about whether
the last argument can be null. All uses changed.
(_GL_HAS_BUILTIN_OVERFLOW_P): Also test __has_builtin.
(_GL_ADD_OVERFLOW, _GL_SUBTRACT_OVERFLOW, _GL_MULTIPLY_OVERFLOW):
Don't try to use 3rd arg null, as this doesn't work on GCC 6.2.0
and it's not clear which GCC versions it works for.
(_GL_INT_OP_WRAPV): Use _GL_HAS_BUILTIN_OVERFLOW instead of
its definiens.