Implement possibility to disable debug output

Description

This adds disabling debug output during runtime when build in Debug mode. Because debugging requires quite a lot of computational power. Default is enabled (print debug messages as before when in Debug mode). DEBUG() calls don't have to be changed.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• ~~Fixes: ~~

When external dependencies are removed

• Reporting to ...