cmake HAS_CXX20 examples

erthink/libmdbx CMakeLists.txt :525 

list(FIND CMAKE_CXX_COMPILE_FEATURES cxx_std_20 HAS_CXX20)

erthink/libmdbx CMakeLists.txt :535 

elseif(NOT HAS_CXX20 LESS 0 AND NOT (CMAKE_COMPILER_IS_CLANG AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10))