Add platform auto-detection

[Sainan]

No description provided.

[alerque]

Better make sure you're not loosing the ability to cross-compile while doing this. That's the usual pitfall of trying to do platform detection at compile time.

[Sainan]

Fair point, although I think it's not a huge concern, e.g. if you're on a Linux machine, you're not gonna be targeting Windows — I'm not sure that would even be possible.

And otherwise I do think the compiler should be setting the correct macros, e.g. if you're on a X86 machine targeting ARM64, MSVC would stop defining _M_IX86 and start defining _M_ARM64.

[alerque]

Is is possible with Lua, that's why I thought to mention it. I build Lua interpreter binaries and various apps with Lua interpreters bundled into them targeting Windows from Linux all the time (see e.g. the luacheck.exe assets on LuaCheck project releases, those are all built on Linux).

[Sainan]

That's interesting. Do you think these changes would break your cross-compiling workflow?