patch 8.0.1709: some non-C89 code may slip through

Problem:    Some non-C89 code may slip through.
Solution:   Enforce C89 in configure. Fix detected problems. (James McCoy,
            closes #2735)
diff --git a/src/version.c b/src/version.c
index 6bebac2..b24935f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -763,6 +763,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1709,
+/**/
     1708,
 /**/
     1707,