The basic idea seems fine, but isn't that off by a factor of 2? It defines size_t_bits_minus_2 = sizeof (size_t) * CHAR_BIT - 2 and then defines SIZE_MAX to (((1U << $size_t_bits_minus_2) - 1) * 2 + 1). Unless I'm missing something, on a 32-bit host, that will set SIZE_MAX to 2147483647 instead of the correct value.
Instead, how about if we compute size_t_bits = sizeof (size_t) * CHAR_BIT and then define SIZE_MAX to (((1U << ($size_t_bits - 1)) - 1) * 2 + 1)?
