could use cmake's CXX_STANDARD to set the C++ standard #257

xtensor-blas, at least in tests in test/CMakeLists.txt and in benchmark/CMakeLists.txt, sets a default C++ standard together with an option (CPP17/CPP20/CPP23, depending on xtensor-blas version) to set a more recent standard.

When the C++ standard, alternative flags are used to allow for GNU, Intel, PGI, or for MSVC compilers.

But cmake already has a mechanism for managing the C++ standard, with the CXX_STANDARD variable, that can be given a custom value at the cmake command line via CMAKE_CXX_STANDARD (with CMAKE_CXX_STANDARD_REQUIRED=ON if it must be strict).

CXX_STANDARD also has a fallback mechanism (a "decay behavior") to drop back to an older standard if the compiler does not support the given standard. Depending on what xtensor-blas wants by setting the standard, it be made strict by using CXX_STANDARD_REQUIRED=ON if needed.

Using these flags puts the burden of tracking the right flag for each compiler onto cmake, so might allow xtensor-blas to work with more rare compilers.

https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html

https://cmake.org/cmake/help/latest/variable/CMAKE_CXX_STANDARD.html#variable:CMAKE_CXX_STANDARD
https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD_REQUIRED.html#prop_tgt:CXX_STANDARD_REQUIRED