CMake language standard for check* functions

A key feature of meta-build systems is dynamically testing system capabilities. Dynamic checks are easier to maintain and use versus large if-else trees or requiring the user to manually include specific Makefiles, etc. In CMake, dynamic testing via check* module functions generally wrap try_compile with logic to only run the check once. try_compile() will run each time CMake runs unless additional if() guard logic is put around try_compile().

check_source_compiles shows the influence six variables CMAKE_REQUIRED* have on the check* functions. Rather than manually setting CMAKE_REQUIRED_FLAGS with compiler language standard flags, instead we recommend setting CMAKE_CXX_STANDARD or similar variable for the code language being checked. The CMAKE_CXX_STANDARD (or similar for C etc.) takes effect in the check* functions.