clang++ and libstdc++ 9 build failure

[DavidTruby]

std::get for std::variant in libstdc++ 9 is erroneously marked noexcept in one place and not marked somewhere else in the same header. GCC allows this but clang does not as the standard forbids it.

This has been fixed upstream so won't be a problem in libstdc++ 10. However I don't think there's an easy workaround for this, so possibly we just need to specify in the Readme that this combination of compiler and standard library won't work.

[DavidTruby]

Here's the upstream libstdc++ bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90397

[RichBarton-Arm]

#1059 has fixed this problem for us. :-)