Add local .gitignore file

Good day,

Would you consider pulling in this .gitignore file? This project creates a fair bit of generated, non-tracked files and this .gitignore file causes any invocation of 'git status' to be much cleaner.

Note: I do not believe this .gitignore file causes git to ignore ALL possible generated files, only the ones that my particular build/use process generates. E.g. I don't used gtags so I am not sure what extra files gtags creates.

Also note: In two sub-directories (config/ and build-aux/) there are some tracked files and some untracked files. So, instead of ignoring the directories completely I have only explicitly ignored the currently untracked file.

Thanks again for your consideration.

Ethan Alan