From the Debian wiki: https://wiki.debian.org/X32Port

X32 is an ABI for amd64/x86_64 CPUs using 32-bit integers, longs
and pointers. The idea is to combine the smaller memory and cache
footprint from 32-bit data types with the larger register set of
x86_64. The 64-bit registers can make computation more efficient,
and with 8 additional registers available, there is less pressure
compared to i386/i686.

rapidjson makes an incorrect assumption in a check for 64 bit
platforms, and uses __LP64__ exclusively. This fix adds an
additional check for __x86_64__ && __ILP32__ defines, as a very
conservative fix. However, the usage of __LP64__ would be a problem
for other "mixed" applications like ARM ILP32, so a better detection
scheme might be needed in the future.