configure: improve FreeBSD DCO check

configure: improve FreeBSD DCO check

The libnv check doesn't work as expected on FreeBSD 14.x, because
FreeBSD has namespaced libnv to avoid conflicts with libnvpair.
This means that the naive check generated by AC_CHECK_LIB() fails to
detect libnv even though it's present.

Instead check for the if_ovpn.h header. This is a more accurate check
anyway, as libnv is present on FreeBSD versions prior to 14 (which do
not support DCO).

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230301091848.80760-1-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26314.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6f261673dee26ae8cfdf58f77038098d4f81d84a)