patch 8.2.3510: changes are only detected with one second accuracy

Problem:    Changes are only detected with one second accuracy.
Solution:   Use the nanosecond time if possible.  (Leah Neukirchen,
            closes #8873, closes #8875)
diff --git a/src/config.h.in b/src/config.h.in
index 0808cc3..61b8b76 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -144,6 +144,9 @@
 /* Define if stat() ignores a trailing slash */
 #undef STAT_IGNORES_SLASH
 
+/* Define to nanoseconds field of struct stat */
+#undef ST_MTIM_NSEC
+
 /* Define if tgetstr() has a second argument that is (char *) */
 #undef TGETSTR_CHAR_P