gcc/gcc.c has a comment at the top that's a guide to adding a command line option.  It's incomplete: it should say to add the new option to c.opt and rebuild the compiler, and then describe how the variable OPT_whatever gets created by the awk scripts during the build process, so you can then check for the option elsewhere in the compiler.  That mechanism doesn't appear documented at all, and took me a while to figure out through code tracing.  Some description in the comment would have been a big help.

Also, the comment at the top of the automatically-generated options.c file says it's generated by "opts.sh" rather than by those awk scripts.  That's a minor thing but updating it wouldn't hurt.