Update to new upstream bugfix release 2.4.6.

While here, warn and sleep for 10 s when building against LibreSSL.

Remove some cruft.

Change summary:
https://github.com/OpenVPN/openvpn/blob/release/2.4/Changes.rst#version-246

Changelog:
https://community.openvpn.net/openvpn/wiki/ChangesInOpenvpn24#OpenVPN2.4.6

Reported by: portscout