Fix 'engine' unit test on FreeBSD (specifically 'not GNU make')

Fix 'engine' unit test on FreeBSD (specifically 'not GNU make')

The rules to generate $(builddir)/openssl.cnf from $(srcdir)/openssl.cnf.in
only worked for GNU Make.  BSD make needs the rules more explicit, and
the target must not have a directory specification (fixes commit
542c69c37).

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Message-Id: <20200629175109.94276-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20159.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>