[Windows] don't use invalid flags on Windows

[SamGondelman]

Summary

This is the fourth PR in my attempt to get Windows builds working. Since "-Wno-missing-prototypes", #pragma GCC diagnostic push, etc. aren't valid with non-gcc compilers, let's wrap them so we are only doing them on non-Windows platforms.

Test plan

Since we aren't actually building on Windows yet, this should have no effect on existing platforms, so just confirm that everything builds.

[pytorch-bot]

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[SS-JIA]

Overall, I think we only need to change the .bzl files to make sure that we aren't using unsupported compile flags for windows. I would try to avoid modifying the .h/.cpp/ files if possible, except to add #ifdef _GNUC_ guards wherever necessary.

[SamGondelman]

@SS-JIA thank you! I've made those changes and it's much better.

If I grep for compile_flags = I'm noticing now that there are several other places that might need the select for Windows...since we aren't actually building on Windows yet, I'm hesitant to go in and change all of them yet for fear of breaking something. what do you think?

[SS-JIA]

@SamGondelman LGTM! Yes, we can update compile flags incrementally.