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/config.h.in b/src/config.h.in
index 530c082..3ff4605 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -209,6 +209,7 @@
 #undef HAVE_CANBERRA
 #undef HAVE_SODIUM
 #undef HAVE_ST_BLKSIZE
+#undef HAVE_SYNC
 #undef HAVE_SYSCONF
 #undef HAVE_SYSCTL
 #undef HAVE_SYSINFO