Fix building of utf8.hpp on clang-10 / libstdc++-8

[jcelerier]

e.g. on Ubuntu 18.04: __cpp_char8_t is indeed >= 201811 as clang-10 already supports a C++20 mode with char8_t, but it uses by default libstdc++'s standard library there which did not provide std::u8string_view at this time

[hanickadot]

Please provide changed single-header libraries too, you can do it with make single-header(you will probably need to install pip package quom)

[hanickadot]

also, the change break some of builds, please look at the failures and try to find a workaround

[jcelerier]

should be fixed now, one build remains when GH can run it: https://github.com/jcelerier/compile-time-regular-expressions/runs/7834679637?check_suite_focus=true

[hanickadot]

thanks!

[jcelerier]

thank you :)