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

c31cbe7 Add C++17 test to Travis (Pieter Wuille)
7829685 Add configure option for c++17 (Pieter Wuille)
0fbde48 Support conversion between Spans of compatible types (Pieter Wuille)
7cbfebb 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 #16684.

ACKs for top commit:
  elichai:
    tACK c31cbe7
  practicalswift:
    Tested ACK c31cbe7
  hebasto:
    ACK c31cbe7, tested on Linux Mint 19.3 both C++11 and C++17 modes. Compiled and passed tests locally.

Tree-SHA512: a4b00776dbceef9c12abbb404c6bcd48f7916ce24c8c7a14116355f64e817578b7fcddbedd5ce435322319d1e4de43429b68553f4d96d970c308fe3e3e59b9d1

.travis.yml

@@ -148,6 +148,11 @@ jobs:
       env: >-
         FILE_ENV="./ci/test/00_setup_env_native_valgrind.sh"
 
+    - stage: test
+      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [no depends, only system libs, c++17]'
+      env: >-
+        FILE_ENV="./ci/test/00_setup_env_native_cxx17.sh"
+
     - stage: test
       name: 'x86_64 Linux  [GOAL: install]  [focal]  [no depends, only system libs, sanitizers: fuzzer,address,undefined]'
       env: >-