pkg: depend on OpenVPN 2.4 for the next -devel build

Now that tools can build several versions of the same software, we
can have -devel depend on a different one than the release.  This
helps with prebuilt testing and easier testing for interested parties.

Should your manual upgrade break at the moment, the easiest solution is:

    # make upgrade CORE_OPENVPN=23

Makefile

@@ -44,7 +44,7 @@ CORE_HASH=	${CORE_COMMIT:C/^.*-//1}
 CORE_ABI?=	17.1
 CORE_ARCH?=	${ARCH}
 CORE_BIND?=	911
-CORE_OPENVPN?=	23
+CORE_OPENVPN?=	# empty for version 2.4
 CORE_PHP?=	70
 CORE_PY?=	27