Add .gitignore file

This .gitignore file contains only untracked files generated by the
build system. For other files depending on your environment, e.g.,
temporary files or backup files create by your editor, you can set
"core.excludesfile" for the global setting, or add lines to
.git/info/exclude file for the local per-repository setting.
(cf. https://help.github.com/articles/ignoring-files)