patch 8.1.2316: FORTIFY_SOURCE can also be present in CPPFLAGS

Problem:    FORTIFY_SOURCE can also be present in CPPFLAGS.
Solution:   Remove it in configure. (Benedikt Morbach, closes #2786)
diff --git a/src/version.c b/src/version.c
index b6a209d..aae3fae 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2316,
+/**/
     2315,
 /**/
     2314,