gitignore another (temporary) autoconf file

[alexjbest]

This shows up on my system when configuring (but only temporarily), nevertheless it is helpful to ignore it in case we accidentally git add * at precisely the wrong moment without noticing

📚 Description

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[fchapoton]

hhhm, not sure if this is very useful, but why not.