clang __cpp_coroutines check undefined error

[ksherlock]

With -Werror and -Wundef, the clang __cpp_coroutines check can cause an error. (error: '__cpp_coroutines' is not defined, evaluates to 0 [-Werror,-Wundef]) Presumably the gcc __cpp_impl_coroutine checks would also suffer from this issue.

Error encountered with Apple clang version 11.0.0 (clang-1100.0.33.17)

[viccpp]

This style is recommended by the Committee - https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations. So I think usage of -Wundef + -Werror is a bad idea in general