Support default values for preprocessor definitions

[eyalroz]

At the moment, if a preprocessor definition was not specified on the command line, using a default requires quite a bit of code in the kernel adapter:

virtual void generate_additional_preprocessor_definitions(execution_context_t& context) const
{
    auto my_def = safe_get_defined_value<whatever>(context.options.preprocessor_definitions.valued, "MY_DEF");
    if (not my_def) {
        context.preprocessor_definitions.generated.valued["MY_DEF"] = default_value_of_my_def;
    }
}

instead, it would be nice if we could simply specify the default value in the parameter definition.