patch 9.1.0842: not checking for the sync() systemcall

Problem:  not checking for the sync() systemcall
Solution: check for sync in configure script, fix related #ifdefs
          (Jonas Termansen)

It's better to check for features directly rather than maintaining a
denylist of operating systems without them.

closes: #15985

Signed-off-by: Jonas 'Sortie' Termansen <sortie@maxsi.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/version.c b/src/version.c
index 3c18aa4..7d02e6c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    842,
+/**/
     841,
 /**/
     840,