Fixes for the build system

1. Fix make distcheck and test it with the Buildbot.
2. Restore ./config/default (still needs to be updated for the new software environment on Breeze).
3. Enable the maintainer mode by default. Let's see whether we get any problems with this (see also here).

added 2 commits

• 07600c9a - Update CI: update Levante detection.
• eee762d2 - Update CI: update CFGLAGS for the ICON integration test.

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added 1 commit

• 4ad71a01 - Update CI: drop support for Debian Stretch on Breeze.

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added 1 commit

• 563d38a1 - Update CI: avoid parallel access to lustre on Levante.

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mentioned in merge request !19 (closed)

merged