patch 8.1.2347: MacOS: build fails

Problem:    MacOS: build fails.
Solution:   Don't define _XOPEN_SOURCE for Mac.
diff --git a/src/version.c b/src/version.c
index b47808d..981f92f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -738,6 +738,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2347,
+/**/
     2346,
 /**/
     2345,