Utility: add a macro for conditional compilation

This is useful for Magnum vectors.

src/Corrade/Utility/Macros.h

@@ -394,12 +394,18 @@ Expands to @cpp constexpr @ce on C++14 and newer, empty on C++11. Useful for
 selectively marking functions that make use of C++14 relaxed constexpr rules.
 @see @ref CORRADE_CXX_STANDARD
 */
-#if CORRADE_CXX_STANDARD >= 201402
+#if __cpp_constexpr >= 201304
 #define CORRADE_CONSTEXPR14 constexpr
 #else
 #define CORRADE_CONSTEXPR14
 #endif
 
+#if defined(__clang__) && __clang_major__ >= 9 || defined(__GNUG__) && __GNUG__ >= 9 || defined(_MSC_VER) && _MSC_VER >= 1931
+#define CORRADE_CONSTEVAL (__builtin_is_constant_evaluated())
+#elif CORRADE_CXX_STANDARD >= 202002L
+#define CORRADE_CONSTEVAL (std::is_constant_evaluated()) /*include type_traits*/
+#endif
+
 /** @hideinitializer
 @brief Always inline a function
 @m_since{2019,10}