#ifdef for 32-bit platform

I recommend bookmarking the predef SourceForge. There's no one answer, but it can certainly help you get started.

EDIT: For GCC-only code, you can use __i386__ to check for 32-bit x86 chips, and I suggest trying __X86_64__ or something similar to check for 64-bit x86 chips. (Note: It has come to my attention that the previous answer involving __ia86__ is actually a different chip, not a 64-bit x86 chip. This just shows my lack of hardware experience. For those more knowledgeable about hardware than I, consule the SourceForge page on predefined macros that I link to above. It's much more accurate than I am.) There are some other ones that would work, but those two should be fairly universal amongs GCC versions.

I'm not sure if there is a universal #if def that is appropriate. The C++ standard almost certainly does not define one. There are certainly platform spcefic ones though.

For example, Windows

#if _WIN64 
// 64 bit build
#else
// 32 bit build
#endif

EDIT OP mentioned this is a cross compile between Windows and Non-Windows using GCC and other compilers

There is no universal macro that can be used for all platforms and compilers. A little bit of preprocessor magic though can do the trick. Assuming you're only working on x86 and amd64 chips the following should do the trick. It can easily be expanded for other platforms though

#if _WIN64 || __amd64__
#define PORTABLE_64_BIT
#else
#define PORTABLE_32_BIT
#endif