Enable feature test macros

clang-tidy doesn't seem to respect feature-test macros, but part of the design is done so that clang-tidy doesn't see through attempts to force a compile-time error (e.g. not using `static_assert` in the `consteval` function). This means that `__cpp_lib_is_constant_evaluated` can't be enabled till clang-tidy acknowledges that.

`consteval` doesn't have a feature-test macro at present.