Ensure STDCALL/STDCALLPTR is only invoked on x86

[GravisZro]

Prevent the code from trying to use stdcall for anything except (32-bit) x86 builds because it's the only platform it's applicable for.

This clears up a lot of annoying warning messages.

Alteration to CMakeList.txt is only whitespace clean up.

[winterheart]

Why just not remove STDCALLs entirely? Is there low-level magic that may prevent it runs in 32bit mode?

[GravisZro]

@winterheart x86 calls had several different forms and without specifying how an exposed library function should be structured your compiler may end up structuring or calling that library function incorrectly.

[GravisZro]

@winterheart
You can learn more about why stdcall is needed here: https://en.wikipedia.org/wiki/X86_calling_conventions

[Lgt2x]

I'll let others review this one, I'm not very comfortable with x86/win32 subjects :)

[winterheart]

I'd suggest move 32-bitness check into configuration level to CMakeLists.txt with (place it after project(...)):

if(CMAKE_SIZEOF_VOID_P EQUAL 4)
    add_definitions("-DD3_ENABLE_STDCALLS")
endif()

And to code goes (lib/module.h or into libraries itself):

#ifdef D3_ENABLE_STDCALLS
#ifdef _MSC_VER // Visual C++ Build
#define STDCALL __stdcall
#define STDCALLPTR *STDCALL
#else // Non-Visual C++ Build
#define STDCALL __attribute__((stdcall))
#define STDCALLPTR STDCALL *
#endif // Visual C++ Build
#else // D3_ENABLE_STDCALLS
#define STDCALL
#define STDCALLPTR STDCALL *
#endif // D3_ENABLE_STDCALLS

And this code will be activated only on 32-bit systems and depends on compiler.

Still, I don't know why this still needed on 32-bit Linux systems, but for compatibility I would left it as is.

[GravisZro]

Basing the architecture on CMAKE_SIZEOF_VOID_P will trigger ANY 32-bit build, including x86, ARMv7, PPC, et ceterea.

I hoped CMake would have a good way to the target architecture but it's OS specific.

As for leaving the STDCALL code "as is", doing so will result in build failures for non-x86/x86_64 targets.

I'll update the code.

[GravisZro]

@winterheart, I have revised the code to exclude non-C++17 compilers and removed duplicate detection macros. I hope this is now sufficiently terse.