Merge #18591: Add C++17 build to Travis

c31cbe7cfe Add C++17 test to Travis (Pieter Wuille)
7829685e27 Add configure option for c++17 (Pieter Wuille)
0fbde488b2 Support conversion between Spans of compatible types (Pieter Wuille)
7cbfebbf3d Update ax_cxx_compile_stdcxx.m4 (Pieter Wuille)

Pull request description:

  This adds a `--enable-c++17` option to the configure script, fixes the only C++17 incompatibility (with a commit taken from #18468), and adds a Travis test for it.

  This is all off by default, and release builds remain C++11.

  It implements the first step of the plan in https://github.com/bitcoin/bitcoin/issues/16684.

ACKs for top commit:
  elichai:
    tACK c31cbe7cfe
  practicalswift:
    Tested ACK c31cbe7cfe
  hebasto:
    ACK c31cbe7cfe, tested on Linux Mint 19.3 both C++11 and C++17 modes. Compiled and passed tests locally.

Tree-SHA512: a4b00776dbceef9c12abbb404c6bcd48f7916ce24c8c7a14116355f64e817578b7fcddbedd5ce435322319d1e4de43429b68553f4d96d970c308fe3e3e59b9d1

ci/test/00_setup_env_native_cxx17.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+export LC_ALL=C.UTF-8
+
+export CONTAINER_NAME=ci_native_cxx17
+export PACKAGES="python3-zmq libevent-dev bsdmainutils libboost-system-dev libboost-filesystem-dev libboost-test-dev libboost-thread-dev libdb5.3++-dev libminiupnpc-dev libzmq3-dev"
+export NO_DEPENDS=1
+export GOAL="install"
+export BITCOIN_CONFIG="--enable-zmq --with-incompatible-bdb --enable-c++17"