I've spelled "HAS_INT64_TYPE" as follows:

+#if SIZEOF_LONG == 8 || SIZEOF_LONG_LONG == 8
+#if SIZEOF_LONG == 8
+    typedef unsigned long npy_uint64;
+#else
+#if SIZEOF_LONG_LONG == 8
+    typedef unsigned long long npy_uint64;
+#endif
+#endif
...

I left the Unpack2 function using floats, since there's no rounding issues there, and it didn't feel like the extra code complexity was justified.  If there's disagreement on that point, I can make a similar change for Unpack2.