gitignore: use absolute path

The current content of `.gitignore` is using relative paths. For
example, `test/config.log` would be ignored while it doesn't seem to be
the intent. Use absolute paths. They are still relative to the root of
the repository.
gitignore: add `test-driver` and `compile`

Those are generated by latest versions of autotools.
Merge pull request #73 from vincentbernat/fix/git-ignore

Small gitignore fixes