Add function to treat compiler warnings as errors

There needs to be a function, cx_cxx_warnings_as_errors and cx_c_warnings_as_errors, to treat compiler warnings as errors. At least one CMake build type should be enabled.

cx_c_warnings_as_errors([DEBUG] [Release] [RelWithDebInfo] [MinSizeRel])

cx_cxx_warnings_as_errors([DEBUG] [Release] [RelWithDebInfo] [MinSizeRel])