Enhance gcc output to show warnings enabled by default

This patch series enhances the GCC options parser to indicate the options that are enabled by default, for the unique and top-level outputs. Having this information indicated within a diff makes it easier to see which options to consider enabling explicitly in a project.

The clang input doesn't appear to have anything corresponding.

A few questions came to mind during implementation -- comments on these or anything else are welcome.

• Should this change in output be enabled by a command line option?
• Do you see any problems or omissions in the heuristic used by DefaultsListener?
• Should the listeners be focused on individual properties and the combination moved from DefaultsListener to parse_options_file?

Looks interesting and promising and would solve some questions that compiler version updates raise. I'll take a closer look when I can focus a little bit on this.