patch 8.0.1236: Mac features are confusing

Problem:    Mac features are confusing.
Solution:   Make feature names more consistent, add "osxdarwin". Rename
            feature flags, cleanup Mac code. (Kazunobu Kuriyama, closes #2178)
diff --git a/src/config.h.in b/src/config.h.in
index fe68d49..e692d40 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -212,6 +212,7 @@
 #undef HAVE_USLEEP
 #undef HAVE_UTIME
 #undef HAVE_BIND_TEXTDOMAIN_CODESET
+#undef HAVE_MBLEN
 
 /* Define, if needed, for accessing large files. */
 #undef _LARGE_FILES