build: update AX_CXX_COMPILE_STDCXX to serial 18

[patricklodder]

Downloaded the latest version available from https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html at the time of writing. This allows configuring for up to c++20.

Prerequisite for getting c++17 to work because the current version (4) only supports up to c++14.

[patricklodder]

rebased to trigger a c++14 CI run

[patricklodder]

Note: if we want to solve #3547, we'll need this to enable support for c++17 through autoconf.

[xanimo]

utACK. no differences from src and dst

[chromatic]

All tests pass and code looks good, but in my local checkout I see that the name Jörn is somehow misrendered. I see the same in the diff as shown on GitHub; is there a missing file encoding we need to add?

[patricklodder]

[..] in my local checkout I see that the name Jörn is somehow misrendered. I see the same in the diff as shown on GitHub; is there a missing file encoding we need to add?

This appears to be an upstream issue, not ours, because the same character sequence is shown in GNU's repo

I propose to keep it the same as upstream - we get better traceability that way, and it's not up to us what the copyright line should say anyway.

[chromatic]

Agreed on matching upstream.

[chromatic]

ACK, all tests pass on x86-64 Linux. Text matches against upstream (with the caveat of potential encoding oddness, though that also reproduces against upstream Git).