patch 8.1.1313: warnings for using localtime() and ctime()

Problem:    Warnings for using localtime() and ctime().
Solution:   Use localtime_r() if available.  Avoid using ctime().
diff --git a/src/config.h.in b/src/config.h.in
index 3f122fd..23e301c 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -174,6 +174,7 @@
 #undef HAVE_GETTIMEOFDAY
 #undef HAVE_GETWD
 #undef HAVE_ICONV
+#undef HAVE_LOCALTIME_R
 #undef HAVE_LSTAT
 #undef HAVE_MEMSET
 #undef HAVE_MKDTEMP