configure.ac: Add a check for memrchr

While we're not using memrchr directly, the compat.h file we indirectly
include from syslog-ng does. To make things compile properly on certain
platforms, like FreeBSD, we need to explicitly check for this too.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>