adding -Wno-c++20-compat flag to CMakeLists.txt

[jdnixx]

(on Kubuntu 23.10) compilation fails at the make -j$(nproc) step with a bunch of warnings

error: identifier ‘char8_t’ is a keyword in C++20 [-Werror=c++20-compat]
  369 | enum char8_t : unsigned char {};
      |      ^~~~~~~

Adding the -Wno-c++20-compat flag resolves it & lets it compile. Note this is only after the changes to cpptoml and cxxopts files with #include <limits> as you said in #17 , so that will have to be addressed too for it to compile out of the box, but I found even after doing that the compatibility thing prevented me from building.