> I don't understand why "#ifdef HAVE_UINT32_T" is tested, since configure ensures that uint32_t is always defined.

Take a look at the explanations in the autoconf file and in pyport.h.  No, configure does *not* always ensure that uint32_t is defined:  it does that only if the platform *doesn't* provide uint32_t, but does provide a 32-bit exact-width unsigned integer type (two's complement, no padding bits, etc. etc.).  Which is why we need to make a second check for the case that the platform *does* define uint32_t directly.