configure: Disable IKEv1 support by default

The protocol has long been replaced by IKEv2 and is officially deprecated
since several years (RFC 9395).  As a first step to removing support for
it completely, this makes the configure option disabled by default and
warns users about its use.

testing/scripts/recipes/013_strongswan.mk

@@ -22,6 +22,7 @@ CONFIG_OPTS = \
 	--with-strongswan-conf=/etc/strongswan.conf.testing \
 	--with-random-device=/dev/urandom \
 	--disable-load-warning \
+	--enable-ikev1 \
 	--enable-curl \
 	--enable-ldap \
 	--enable-eap-aka \