set __cdist_dry_run env var if dry-run

See merge request ungleich-public/cdist!781

Re-arrange Makefile and build-helper script

See merge request ungleich-public/cdist!778

Maintainers should use build-helper script.
End users should use Makefile, which contains targets
that can be run on pure source (without git repository).

Add 'Perils of CDIST_ORDER_DEPENDENCY' sub-section

See merge request ungleich-public/cdist!777

Cdist 5.0.0

See merge request ungleich-public/cdist!775

Speeches should be generated on demand when needed
and the cdist website should be updated.

By default, was getting the following error:
curl: (92) HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)

If it fails then the whole release should not fail,
signing git tag can be tried again.

Detect dependency cycle as soon as possible

See merge request ungleich-public/cdist!774