Updated CMake and used [[maybe_unused]] if available.

[wermos]

In the top-level, CMakeLists.txt I specificed the project language and also put the C++ version language as 14 because in Fastor/config/config.h, the compilation is set to error out if the compiler doesn't at least support C++14.

I also modified the test for C++20 because after the release of C++20, the proper way to check for support is to check if __cplusplus is defined to be 202002L or not.

Finally, I changed the use of the [[gnu::unused]] to [[maybe_unused]] if available and compiler-specific pragmas otherwise, because [[gnu::unused]] was emitting error C5030 on MSVC v19.35 (attribute 'gnu::unused' is not recognized).

[wermos]

Currently, there is no way to have CMake do something like "use C++20 if available, but if it's not, then fallback to 17, and if that's not available, fall back to 14, and if that's not available, then abort compilation." This "decaying" behavior is available, but it's all or nothing: If we enable it (which is what it's currently doing), then it keeps decaying until it finds a version that the compiler supports (which might be, say, C++11 or even C++98).

There is however, one thing we can do: set the value of CMAKE_CXX_STANDARD to 20, and allow CMake to decay it. Coupled with the aborting behavior defined in Fastor/config/config.h, this should be able to achieve the behavior we want.

[wermos]

Not sure why the build is failing. What version of C++ does "Visual Studio 16 2019" have?