Check whether the CXX compiler supports a given flag.


<flag> - the compiler flag
<var>  - variable to store the result

This internally calls the check_cxx_source_compiles macro and sets CMAKE_REQUIRED_DEFINITIONS to <flag>. See help for CheckCXXSourceCompiles for a listing of variables that can otherwise modify the build. The result only tells that the compiler does not give an error message when it encounters the flag. If the flag has any effect or even a specific one is beyond the scope of this module.