Boost headers try to include experimental/string_view when std is less
than c++17. This does not work ith libcxx where this header is not
present.

Refreshed patches.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Kea is an open source DHCPv4/DHCPv6 server being developed by
Internet Systems Consortium. Kea is a high-performance, extensible
DHCP server engine that is designed to be easily modified and extended
with hooks libraries.

Kea is free open source, and we welcome community engagement, via the
Kea-users mailing list, this wiki, and our Github
repository. There is a small core team of dedicated software engineers
developing it and we need your contributions and support
contracts to support them.

DHCP Standardization efforts: The lead developer on KEA is
co-chair of the Dynamic Host Configuration working group in the
IETF. We are committed to providing a standards-compliant
implementation and are closely tracking developments in this working
group and evaluating them for inclusion in KEA.

wiki : http://kea.isc.org/wiki
official : https://www.isc.org/kea/

Signed-off-by: BangLang Huang <banglang.huang@foxmail.com>