Check whether the C compiler supports a given flag.


<flag> - the compiler flag
<var>  - variable to store the result
         Will be created as an internal cache variable.

This internally calls the check_c_source_compiles macro and sets CMAKE_REQUIRED_DEFINITIONS to <flag>. See help for CheckCSourceCompiles 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.