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/evalfunc.c b/src/evalfunc.c
index f3b1faf..ff51d12 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -13213,6 +13213,9 @@
 f_strftime(typval_T *argvars, typval_T *rettv)
 {
     char_u	result_buf[256];
+# ifdef HAVE_LOCALTIME_R
+    struct tm	tmval;
+# endif
     struct tm	*curtime;
     time_t	seconds;
     char_u	*p;
@@ -13224,7 +13227,11 @@
 	seconds = time(NULL);
     else
 	seconds = (time_t)tv_get_number(&argvars[1]);
+# ifdef HAVE_LOCALTIME_R
+    curtime = localtime_r(&seconds, &tmval);
+# else
     curtime = localtime(&seconds);
+# endif
     /* MSVC returns NULL for an invalid value of seconds. */
     if (curtime == NULL)
 	rettv->vval.v_string = vim_strsave((char_u *)_("(Invalid)"));