Requiring CMAKE_CXX_STANDARD is a bug, not a feature

[jonesmz]

CMAKE_CXX_STANDARD is not supposed to be used the way that toml11 is trying to use it. This variable represents the default standard version of C++ that targets will be compiled for if they are not otherwise told to compile for a specific C++ standard.

I have several CMake based codebases with dozens of targets, and different targets are compiled with different versions of the C++ standard. These codebases do not set CMAKE_CXX_STANDARD at all, because each target has their own version.

Please revert this commit, https://github.com/ToruNiina/toml11/commit/4c4e82866ecd042856b7123a83f80c0ad80a5e13, it breaks projects that use per-target property CXX_STANDARD.