Add CMake feature test for lambdas in unevaluated context

In order to use C++20 concepts in the CI, we need to check if the compiler is able to declare lambdas in unevaluated contexts. Since there is no feature macro for it even in c++20, this MR adds a CMake check that gives us this information in the config.h header. See dune-grid!377 (comment 90349)

mentioned in merge request dune-grid!377 (merged)

changed title from Add CMake feature test unevaluated context lambda to Add CMake feature test for lambdas in unevaluated context

added feature label

changed the description

Looks good to me. We need this for parting some of the concept checks to dune core modules.

merged

mentioned in commit 7bbf79a5