locale.c: Create a convenience #define

This takes this #ifdef combination and creates a single symbol that
means essentially the same thing.  This is in preparation for it being
used in additional places.  And it makes it clearer what is meant.

It also adds another condition which isn't currently tested for, but has
the same meaning, and doesn't affect the current usage, but will future
ones.

Author: khwilliamson

locale.c

@@ -156,6 +156,15 @@ static int debug_initialization = 0;
 #  include <wctype.h>
 #endif
 
+/* If we have any of these library functions, we can reliably determine is a
+ * locale is a UTF-8 one or not.  And if we aren't using locales at all, we act
+ * as if everything is the C locale, so the answer there is always "No, it
+ * isn't UTF-8"; this too is reliably accurate */
+#if   defined(HAS_SOME_LANGINFO) || defined(HAS_MBTOWC) || defined(HAS_MBRTOWC) \
+ || ! defined(USE_LOCALE)
+#  define HAS_RELIABLE_UTF8NESS_DETERMINATION
+#endif
+
 #ifdef USE_LOCALE
 
 PERL_STATIC_INLINE const char *
@@ -2978,7 +2987,7 @@ S_get_locale_string_utf8ness_i(pTHX_ const char * locale,
         }
     }
 
-#  if defined(HAS_SOME_LANGINFO) || defined(HAS_MBTOWC) || defined(HAS_MBRTOWC)
+#  ifdef HAS_RELIABLE_UTF8NESS_DETERMINATION
 
     /* Here, we have available the libc functions that can be used to
      * accurately determine the UTF8ness of the underlying locale.  If it is a