intprops: add safe, portable integer overflow checking

intprops: add safe, portable integer overflow checking

* lib/intprops.h (_GL_INT_CONVERT, _GL_INT_TWOS_COMPLEMENT):
(_GL_INT_SIGNED, _GL_INT_MINIMUM, _GL_INT_MAXIMUM):
(_GL_SIGNED_INT_MINIMUM, INT_ADD_RANGE_OVERFLOW):
(INT__SUBTRACT__RANGE_OVERFLOW, INT_NEGATE_RANGE_OVERFLOW):
(INT_MULTIPLY_RANGE_OVERFLOW, INT_REMAINDER_RANGE_OVERFLOW):
(INT_LEFT_SHIFT_RANGE_OVERFLOW, _GL_ADD_OVERFLOW):
(_GL__SUBTRACT__OVERFLOW, _GL_MULTIPLY_OVERFLOW, _GL_DIVIDE_OVERFLOW):
(_GL_REMAINDER_OVERFLOW, _GL_UNSIGNED_NEG_MULTIPLE, INT_ADD_OVERFLOW):
(INT__SUBTRACT__OVERFLOW, INT_NEGATE_OVERFLOW, INT_MULTIPLY_OVERFLOW):
(INT_DIVIDE_OVERFLOW, INT_REMAINDER_OVERFLOW):
(INT_LEFT_SHIFT_OVERFLOW, _GL_BINARY_OP_OVERFLOW): New macros.