Use a macro for when __has_cpp_attribute is unsupported

And the standard macro is __has_cpp_attribute, not __has_attribute.

common/opthelpers.h

@@ -11,6 +11,12 @@
 #define HAS_BUILTIN(x) (0)
 #endif
 
+#ifdef __has_cpp_attribute
+#define HAS_ATTRIBUTE __has_cpp_attribute
+#else
+#define HAS_ATTRIBUTE(x) (0)
+#endif
+
 #ifdef __GNUC__
 #define force_inline [[gnu::always_inline]] inline
 #elif defined(_MSC_VER)
@@ -39,12 +45,12 @@
 /* This shouldn't be needed since unknown attributes are ignored, but older
  * versions of GCC choke on the attribute syntax in certain situations.
  */
-#if !__has_attribute(likely)
-#define LIKELY
-#define UNLIKELY
-#else
+#if HAS_ATTRIBUTE(likely)
 #define LIKELY [[likely]]
 #define UNLIKELY [[unlikely]]
+#else
+#define LIKELY
+#define UNLIKELY
 #endif
 
 namespace al {