common/Compiler: only define language keyword definitions if undefined

src/common/Compiler.h

@@ -226,26 +226,34 @@ definition to detect its implementation was only added in C++17. */
 	#define NOEXCEPT noexcept
 	#define NOEXCEPT_IF(x) noexcept(x)
 	#define NOEXCEPT_EXPR(x) noexcept(x)
-#else
-	#define NOEXCEPT
-	#define NOEXCEPT_IF(x)
-	#define NOEXCEPT_EXPR(x) x
 #endif
 
 // Uses SD-6 Feature Test Recommendations
 #if defined(__cpp_constexpr)
 	#define CONSTEXPR constexpr
 	#define CONSTEXPR_FUNCTION constexpr
-
 	#if __cpp_constexpr >= 201304
 		#define CONSTEXPR_FUNCTION_RELAXED constexpr
-	#else
-		#define CONSTEXPR_FUNCTION_RELAXED
 	#endif
-#else
-	// Work around lack of constexpr
+#endif
+
+// Work around lack of language keywords.
+#if !defined(NOEXCEPT)
+	#define NOEXCEPT
+#endif
+#if !defined(NOEXCEPT_IF)
+	#define NOEXCEPT_IF(x)
+#endif
+#if !defined(NOEXCEPT_EXPR)
+	#define NOEXCEPT_EXPR(x) x
+#endif
+#if !defined(CONSTEXPR)
 	#define CONSTEXPR
+#endif
+#if !defined(CONSTEXPR_FUNCTION)
 	#define CONSTEXPR_FUNCTION
+#endif
+#if !defined(CONSTEXPR_FUNCTION_RELAXED)
 	#define CONSTEXPR_FUNCTION_RELAXED
 #endif