Fix ACL_CHECK_ADD_COMPILE_FLAGS to work with clang

Fix ACL_CHECK_ADD_COMPILE_FLAGS to work with clang

Some compilers (e.g., clang) only issue a warning for
unsupported options unless an additional flag such
as -Werror is used to convert the warning to an error.

The behaviour is unchanged when using gcc as it either
errors or ignores unknown options whether or not -Werror
is present.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1574183023-6136-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19170.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>