Fix importc_compare test on Windows

[pbackus]

MSVC does not support C11 atomics, but it still provides a "stub" version of stdatomic.h, which aborts compilation with an #error directive when included.

To avoid this error, explicitly set the C standard to C11 when invoking the C preprocessor on Windows, and check for the C11 macro __STDC_NO_ATOMICS__ before including stdatomic.h.

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Found in dlang/phobos#10650 (comment)

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