Merge pull request #16547 from pekdon/sunos

Portability improvements, first steps at getting neovim on x86_64 SunOS

src/nvim/message.c

@@ -2054,7 +2054,7 @@ static void msg_puts_display(const char_u *str, int maxlen, int attr, int recurs
       msg_ext_last_attr = attr;
     }
     // Concat pieces with the same highlight
-    size_t len = strnlen((char *)str, maxlen);             // -V781
+    size_t len = STRNLEN(str, maxlen);             // -V781
     ga_concat_len(&msg_ext_last_chunk, (char *)str, len);
     msg_ext_cur_len += len;
     return;