configure.ac: Make ACL_CHECK_ADD_COMPILE_FLAGS append instead of prepend

configure.ac: Make ACL_CHECK_ADD_COMPILE_FLAGS append instead of prepend

The prepend behavior is surprising. If there is a difference
in behavior at least make it easy to understand what happens
why.

Change-Id: I1ce408d4473874d88a348308503527ef7eb8c1ff
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1204
Message-Id: <20250923122006.12212-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59237450/
Signed-off-by: Gert Doering <gert@greenie.muc.de>