lablgtk and sourceview: add troubleshooting post-messages and fix depexts

Not sure if we want to enable this automatically to print hints using the depexts. Could be
nice but we need a reliable way to know the distribution, and what would be the fallback ?
Merge pull request #1327 from AltGr/refs/heads/lablgtk-postmessages

lablgtk and sourceview: add troubleshooting post-messages and fix depexts