Fix C++17 version detection in helper_macros.hpp

[nickjeliopoulos]

It seems that __cplusplus can be inconsistent with _MSVC_LANG when discerning C++17 version in some cases (for header-only includes of CUTLASS). See https://github.com/NVIDIA/cutlass/issues/1474. Added switch to check _MSVC_LANG in addition to __cplusplus.

[jackkosaian]

@mhoemmen, can you take a look?

[d-k-b]

Please set the flag /Zc:__cplusplus as shown in https://github.com/NVIDIA/cutlass/blob/7d49e6c7e2f8896c47f586706e67e1fb215529dc/CMakeLists.txt#L439-453 to get MSVC to set the compile options appropriately.

[nickjeliopoulos]

I was able to include CUTLASS headers successfully with @d-k-b suggestion. I closed my issue #1474 related to this.

Is there still some value to this PR for those who want to include CUTLASS headers-only? Maybe this is better solved upstream (in something like PyTorch's torch/utils/cpp_extensions.py per my solution in #1474, or just leaving it to the user to configure builds more carefully?)

[mhoemmen]

@jackkosaian Hi! Thanks for the heads-up! I can take a look.

[nickjeliopoulos]

Thanks for your review @mhoemmen. I committed your suggestion for the CUTLASS_CPLUSPLUS macro, I think that seems more elegant and is in line with the style of other CUTLASS macros.

[mhoemmen]

@nickjeliopoulos Excellent, thanks so much!

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[mhoemmen]

Let me check on the status of this with @hwu36 .

[mhoemmen]

@hwu36 I think we can merge this.